Kamala Harris often leans on a favored phrase to focus her team before beginning an important project: “What business are we trying to accomplish here?”

In deciding what to say in the most important speech of her life on Thursday, the vice president’s answer has been threefold, aides said: tell her life story, frame her contest with Donald J. Trump as one pitting the future against the past and reclaim the banner of patriotism for the Democratic ticket.

Ms. Harris has been taking her convention address so seriously that she has held rehearsals complete with teleprompters in three different time zones.

Soon after she became a presidential candidate one month ago, she told advisers that she saw this speech and any fall debates as the two most pivotal moments of the abbreviated race, according to three people familiar with her thinking. But in reality, she saw this speech as crucial for even longer than that. The earliest draft of her convention remarks had first circulated back when Ms. Harris was still just a vice president seeking a second term as President Biden’s No. 2.

Now, the reworked address will represent Ms. Harris’s biggest turn on the national stage since her sudden ascent to the top of the Democratic Party as she prepares to take on Mr. Trump in an election just 75 days away.

The preparations over both her message and her delivery have been intensive. Adam Frankel, a former speechwriter for former President Barack Obama and now an adviser to Ms. Harris, is the lead writer of the address, taking input and suggestions from a wide variety of others. But the vice president herself has work-shopped the speech nearly line by line, two people familiar with the preparations say.

Her rehearsals and discussion sessions have involved teleprompters this week in Chicago at the Park Hyatt hotel, at Howard University in Washington and in Arizona during her first swing-state campaign trip, when some advisers traveled to join her at a downtown Phoenix hotel.

“She understands how important it is,” said Cedric Richmond, an adviser to Ms. Harris. “I think she probably overstands how important the moment is.”

The first of the three themes of the speech, according to campaign officials with knowledge of it not authorized to speak publicly before the address, is to tell the story of her own life. Ms. Harris is expected to describe her own middle-class upbringing, framing it as allowing her to better understand the needs and struggles of the middle class today. And she is expected to tell voters about her career before the vice presidency: her early work as a prosecutor who rose to become the attorney general of California.

“This is an opportunity for her to tell the American people who she is,” Brian Nelson, a senior policy adviser to Ms. Harris who has worked with her since her time as attorney general, said on Wednesday at a Bloomberg Newsmaker event.

The second goal is to frame the race as one of future versus past, contrasting her promise to protect freedoms and provide a brighter new chapter with darker warnings of Mr. Trump’s agenda and Project 2025, which Democrats have turned into a catchall cause for concern.

The third is to tap into an appeal to patriotism. The party handed out “U.S.A.” signs to delegates throughout the week, and Ms. Harris is expected to pitch herself as a president for all Americans. “We love this country,” Gov. Tim Walz of Minnesota, Ms. Harris’s running mate, said in his own address on Wednesday.

Ms. Harris’s relatively humble roots have been a recurring theme throughout the convention. Speaker after speaker has referred to her early time working at a McDonald’s to contrast with Mr. Trump’s inherited wealth as a New York developer. “We have a chance to elect a president who is for the middle class because she is from the middle class,” as Representative Alexandria Ocasio-Cortez of New York put it in her speech on Monday.

It is notable that the only major policy area where Ms. Harris has rolled out her own agenda separate from Mr. Biden so far is on the economy, focusing on bread-and-butter issues such as the cost of housing and groceries.

“She’s thinking about how she talks to the American people so they know she understands what they’re going through,” Mr. Richmond said of the speech. “That she cares about what they’re going through, that she wants to offer a solution to what they’re going through and that she’s going to do the hard work to do it.”

In just a few short weeks as a candidate, Ms. Harris has already honed a regular speech on the campaign trail that has inspired eager participation from her crowds. They cheer “we’re not going back” and wait for her quip linking her past as a prosecutor to Mr. Trump: “I know his type.”

But those lines have mostly been delivered to Democratic crowds. And while the convention hall certainly will be filled with those voters, television viewers will offer Ms. Harris the opportunity to appeal to a wider electorate, including independents and open-minded Republicans.

Ms. Harris is trying to walk a complex tightrope to present herself as a new leader for the country even though she is part of the current administration. She is also trying to counter the Trump campaign’s portrayal of her as “dangerously liberal,” and she has backed away from a number of more progressive positions she took during her presidential run in 2020.

Ms. Harris has carved out a reputation as a sharp debater and questioner of witnesses in the Senate more than as a soaring orator.

But Nathan Barankin, her former chief of staff in the Senate, said that was an underestimation of her skills.

“To the extent that people have an impression that delivering speeches is not a strength, that is a product of her reading words other people have put in her mouth,” he said. “When she is directly engaged, she makes the content and delivery better.