As the Nevada caucuses drew to a close in February, Donald J. Trump and several top aides gathered for a quick dinner in a suite atop his hotel in Las Vegas before he descended and declared victory.

But the Republican billionaire at the center of attention during the meal was not Mr. Trump — it was Dr. Miriam Adelson.

The former president, by then on a glide path to the Republican nomination, wanted financial support from Dr. Adelson, a conservative megadonor, and she had a request or two. Be less bombastic, she told him, and speak more directly about the economy. But more important, she made clear to Mr. Trump and his top aides, Susie Wiles and Chris LaCivita, be patient.

She would not donate to him while Nikki Haley, his last rival standing and a friend of hers, remained in the race, according to two people briefed on the meeting who insisted on anonymity to describe a private gathering.

But once the primary was over? She gave Mr. Trump a renewed assurance that the famous Adelson geyser of cash — which had shot out hundreds of millions of dollars over more than a decade — would erupt again.

It would have been uncouth that evening on the Strip to put dollars and cents on it, but Dr. Adelson is now fulfilling her promise, making moves to spend more than $90 million to help Mr. Trump’s third White House bid.

She is poised to become one of the biggest donors in the presidential election — and, if Mr. Trump wins, one of the most powerful private citizens with a say in American foreign policy. Fiercely hawkish on Israel, she was deeply unnerved by the Hamas attacks on Oct. 7 and would be likely to shape a second Trump administration’s posture on the Israeli-Palestinian conflict.

Dr. Adelson, 78, has never been a shrinking violet, but she long operated in the shadow of her husband, Sheldon G. Adelson, who ascended to first-name-only status in Republican circles. Mr. Adelson died at 87 in January 2021, ushering her into a new era: For the first time in presidential politics, she is a solo practitioner.

“I’m pretty sure that her North Star going forward is going to be what she thinks Sheldon would have done if he were still alive,” said David M. Friedman, Mr. Trump’s former ambassador to Israel and a longtime confidant of the couple. He was one of nearly 20 associates of Dr. Adelson’s in business and politics who spoke about her to The New York Times, some of them insisting on anonymity to offer candor.

“Miri,” as friends call Dr. Adelson, a physician who specializes in drug addiction and is known for her distinctive highlighter-blond hair and pink-tinted glasses, is one of the wealthiest women in the world. She is, in some ways, a political carbon copy of her husband: intensely pro-Israel, rabidly partisan, and a believer in the nobility of using her money, north of $30 billion, and her media empire to buy influence and shape the world.

But she is also, by the accounts of people who have pitched her, a tougher ask. She is seen as more cerebral and disciplined. Where Mr. Adelson enjoyed the game of politics and obsessed over tactics — he wasn’t above editing the script of an advertisement, one of the people said — Dr. Adelson is more driven by whatever is happening in American and Israeli news. Her fervor for Mr. Trump, some of his allies say, actually exceeds her late husband’s: At one point, she suggested adding a “Book of Trump” to the Bible.

Unlike the Boston-reared Mr. Adelson, Dr. Adelson was born in Israel and speaks fluent Hebrew, her English marked by a heavy Israeli accent. A former officer in the Israel Defense Forces, she spends most of her time these days not in their longtime home Las Vegas but in Israel, where she holds dual citizenship.

Her Israeli nationalism has tethered her to Mr. Trump, especially since Oct. 7. She has argued that people who criticize Israel or offer only qualified support are “dead to us.”

The Adelsons’ bond with him was etched in stone when his administration moved the United States Embassy in Israel to Jerusalem from Tel Aviv: They sat in the front row at the opening ceremony, and would later pay $88 million for the Mediterranean villa in Herzliya where the ambassador had lived before the embassy move, helping to ensure that a future administration could not easily reverse it. When Dr. Adelson entered the home for the first time after buying it, shortly after her husband’s death, she wore a pair of his ill-fitting shoes, Mr. Friedman recalled her telling him.

This year, Dr. Adelson does not have a specific wishlist for Mr. Trump, three people close to her said, in contrast to when she and her husband implored Mr. Trump to move the embassy. Her spokesman, Andy Abboud, denied a recent report that Dr. Adelson was urging Mr. Trump to publicly support an annexation of the West Bank by the Israeli government in exchange for her backing.

But Shmuley Boteach, a rabbi and a longtime friend of the couple, said he believed Dr. Adelson supported annexation and considered so-called “land for peace” deals to actually be “land for war.”

“Do I believe that Miriam supports the creation of a Palestinian state? Absolutely not,” Mr. Boteach said. “Those of us who are part of her circle and share her values,” he continued, would oppose territorial concessions “if there was any doubt whatsoever that the creation of the Palestinian state would lead to murdered Jews.”

Not everyone in Dr. Adelson’s orbit believes that Mr. Trump will deliver for her if he wins a second term. John R. Bolton, a longtime friend of the Adelsons who saw them meet with Mr. Trump in the Oval Office multiple times, argued that she was bound to be disappointed this time around.

“Trump says anything he thinks he can get away with to the audience at the time,” said Mr. Bolton, who turned against Mr. Trump after serving as his national security adviser. “Anybody who puts stock in what he says in the morning is likely to be disappointed by the afternoon if he says something different to somebody else.”

Dr. Adelson’s bets don’t always pay off. She and her husband enjoyed an extraordinarily close relationship with Prime Minister Benjamin Netanyahu of Israel, who even prevailed upon them to start their own newspaper, Israel Hayom, which enthusiastically promoted his administration. But her relationship with Mr. Netanyahu is now virtually nonexistent, and the paper’s coverage has grown more critical of his leadership.

An Inseparable Conservative Couple

The Adelsons, who were both on their second marriage, were exceptionally close even for a longtime couple. They often operated as a single unit politically: Republican fund-raisers always believed that Dr. Adelson’s buy-in was important for Mr. Adelson to feel comfortable parting with their millions.

Mr. Boteach recalled an afternoon he spent with Dr. Adelson several years ago in New York — in a rare occurrence, without Mr. Adelson.

“They called each other in front of me 20 times in four hours. No one understands just how inseparable they are,” Mr. Boteach said. “And that was the only time that I saw them in 15 years of close friendship ever go somewhere separately. And they were on the phone the whole time.”

Still, some Republicans worried after Mr. Adelson’s death about whether Dr. Adelson would keep up his level of funding. She observed a traditional Jewish year of mourning, largely avoiding politics. Meanwhile, G.O.P. fund-raisers entered a panic, casting far and wide for “the next Sheldon.”

Despite her support for Israel, she had been only lightly involved with the Republican Jewish Coalition, an influential conservative group: Some of its board members today say they do not know her well.

In 2012, the Adelsons’ $20 million investment in a super PAC backing Newt Gingrich’s unsuccessful campaign showed how much the Supreme Court’s Citizens United decision two years earlier had transformed presidential politics. Some people close to the couple think the costly experience deterred them from future involvement in presidential primaries.

In 2016 as in 2024, the Adelsons engaged only in the general election, even though Mr. Trump was hardly their first choice. (Dr. Adelson was partial to Ted Cruz, and Mr. Adelson to Marco Rubio.)

Mr. Boteach introduced the Adelsons to Jared Kushner, Mr. Trump’s son-in-law, and eventually the couple sat down with Mr. Trump himself in late 2015. Mr. Adelson found him “very charming,” and over the next year, the couple would press him and Mr. Kushner about relocating the embassy. Dr. Adelson was familiar with disappointment, though, Mr. Boteach recalled, including when George W. Bush failed to follow through on his promised embassy move.

But Mr. Trump made good on his promise, and went on to award Dr. Adelson the Presidential Medal of Freedom, the nation’s highest civilian honor. By 2020, the Adelsons were full-blown Trump believers, putting upward of $90 million into a pro-Trump super PAC, helmed by Mr. LaCivita, called Preserve America. Then, just a few days after the Capitol riot on Jan. 6, 2021, Mr. Adelson died.

“The world has lost a great man,” Mr. Trump said.

Plans for a ‘Very Generous’ Gift to Trump

Mr. Adelson’s death thrust the spotlight on his wife. As the 2024 election approached, she privately expressed a desire to again not touch the primaries. The Trump team, or at least most of it, understood.

Still, Mr. Trump stayed close. He and Dr. Adelson met for half a dozen dinners across the country in 2023 and 2024 that stretched as long as three hours, talking about Israel and their families. During one dinner, Dr. Adelson memorably gave Mr. Trump a necktie of her husband’s that he had been proud of. The next time they dined, Mr. Trump wore the tie, which touched her.

Around the time they broke bread at Mar-a-Lago in mid-March, Dr. Adelson told a confidant, Mort Klein, the president of the Adelson-funded Zionist Organization of America, that she was planning on making a “very generous” gift to Mr. Trump’s re-election effort, Mr. Klein recalled.

Dr. Adelson has not pledged a certain sum — so far, she has donated less than $10 million to Preserve America, which has been resuscitated to again serve as her main political vehicle, a person with knowledge of the matter said — but her team is planning a budget of about $100 million, or a bit more than the Adelsons spent for Mr. Trump in 2020.

Still, the more patrician Adelsons have not always fit neatly into the freewheeling Trump orbit.

Friends described both as suspicious of operatives they haven’t worked with previously. Dr. Adelson’s team has been dismissive of several of the main pro-Trump outside groups, seeing them as ineffective or untrustworthy. Her allies have been rankled by a few Trump operatives who have made what they saw as overly aggressive efforts to land Dr. Adelson’s money.

The mistrust is mutual. The Adelsons’ eagerness over the years to route their money through their own groups has bothered some pro-Trump organizations that saw the couple’s posture as superfluously duplicative. This time around, Mr. Trump has made disparaging comments about the Adelson-funded super PAC’s more establishment-friendly operatives, according to a person who has heard him.

Ike Perlmutter, the billionaire former chief executive of Marvel Entertainment, started a pro-Trump super PAC, and his team made a particularly forward effort. He spoke with Dr. Adelson, his longtime friend, multiple times over the last year and tried to persuade her to join forces with him, but she ultimately declined. The Adelsons have long had a reputation as lone wolves, and Dr. Adelson was always more likely to donate to a group she could control.

“I was surprised, frankly, by the size” of Dr. Adelson’s planned giving to Mr. Trump, said Michael Leven, the longtime former No. 2 to Mr. Adelson at his company, Las Vegas Sands, and a friend of the couple for decades. He asked, upon reading of the expected number of millions, “What was wrong with 50 or 25?”

Mr. Leven said he considered reports of Dr. Adelson’s expected contributions to be “out of character” for a woman he praised as less impulsive than her late husband. “I saw him cut off certain philanthropic giving when a guy or a woman did something he didn’t like. He’s out. There’s no discussion about it. Her entire discipline would be more — I don’t want to say forgiving, but I’d say more understanding.”

Mr. Leven recalled that Dr. Adelson was one of the few people Mr. Adelson would ever listen to. Her advice could be overruled: She at one point encouraged him to pull the company out of Macau, but he declined. But at another point, during the global financial crisis, she persuaded him to put $1 billion of his money, about a quarter of his net worth, into Las Vegas Sands to save the company, Mr. Leven said.

‘We Will Definitely Be Heard’

Dr. Adelson’s plan for heavy spending is arriving fairly early by her family’s standards: It has a reputation for delivering manna from heaven late in elections, around the fall.

Dave Carney, a Republican strategist whom the Adelsons tapped to lead their super PAC, said that he was now primarily conducting relatively inexpensive survey research, but that he was planning to spend roughly $100 million in three or four yet-to-be-decided states, primarily on television and digital ads. He described his group as essential to leveling the playing field with President Biden.

“Up until recently, there’s been 100 percent spending from the Biden side and 0 percent from the Trump side,” Mr. Carney said. “We will definitely be heard. They will probably spend more money, because they already have a huge head start. But we’re starting the game tied.”

Some other major donors, including those who have opposed Mr. Trump, are seriously considering chipping in for Preserve America, according to three people with knowledge of their conversations.

Politics has hardly been Dr. Adelson’s only concern. She has overseen an aggressive expansion of the Sands empire into Texas, including a plan to build an enormous casino near Dallas. And seemingly out of nowhere, at the end of 2023, she and her son-in-law bought the Dallas Mavericks from Mark Cuban, the team’s longtime owner, but, in an unusual arrangement, allowed the exuberant entrepreneur to stay on in a leadership role.

This month, the Mavericks made their first N.B.A. Finals run in over a decade before losing to the Boston Celtics. Dr. Adelson went to some games this season, and stepped up her attendance during the playoffs, but unlike the omnipresent Mr. Cuban, she is rarely seen on television.

Still, “she has become a huge Mavs fan,” Mr. Cuban wrote in an email before the Finals. “Always wearing Mavs gear. Giving hugs.”

“I’m always happy to see her,” he added. But Mr. Cuban, an avowed liberal, declined to weigh in when asked whether he was happy about his business partner’s politics and her love for Mr. Trump.