President Biden issued an executive order on Tuesday that prevents migrants from seeking asylum at the U.S.-Mexico border when crossings surge, a dramatic election-year move to ease pressure on the immigration system and address a major concern among voters.

The measure is the most restrictive border policy instituted by Mr. Biden, or any other modern Democrat, and echoes an effort in 2018 by President Donald J. Trump to cut off migration that was blocked in federal court.

In remarks at the White House, Mr. Biden said he was forced to take executive action because Republicans had blocked bipartisan legislation that had some of the most significant border security restrictions Congress had considered in years.

“We must face a simple truth,” said the president, who was joined by a group of lawmakers and mayors from border communities. “To protect America as a land that welcomes immigrants, we must first secure the border and secure it now.”

Aware that the policy raised uncomfortable comparisons, Mr. Biden took pains to distinguish his actions from those of Mr. Trump. “We continue to work closely with our Mexican neighbors instead of attacking them,” Mr. Biden said. He said he would never refer to immigrants as “poisoning the blood” of the country, as Mr. Trump has done.

Still, the move shows how drastically the politics of immigration have shifted to the right in the United States. Polls suggest there is support in both parties for border measures once denounced by Democrats and championed by Mr. Trump as the number of people crossing into the country has reached record levels in recent years.

Mr. Biden’s executive action was set to go into effect at 12:01 a.m. on Wednesday, at which point border officers could return migrants across the border into Mexico or to their home countries within hours or days.

The American Civil Liberties Union said it planned to challenge the executive action in court.

“The administration has left us little choice but to sue,” said Lee Gelernt, a lawyer at the A.C.L.U, which led the charge against the Trump administration’s attempt to block asylum in 2018 and resulted in the policy being stopped by federal courts. “It was unlawful under Trump and is no less illegal now.”

Assuming it survives legal challenges, the policy kicks in once the seven-day average for daily illegal crossings hits 2,500 — a regular occurrence now. The border would reopen only after the figure drops to 1,500 for seven days in a row and stays that way for two weeks.

That is a significant shift in how asylum has worked for years.

Typically, migrants who cross illegally and claim asylum are released into the United States to wait for court appearances, where they can plead their cases. But a huge backlog means those cases can take years to come up.

The new system is designed to deter those illegal crossings.

There would be limited exceptions to the restrictions announced Tuesday, including for minors who cross the border alone, victims of human trafficking and those who use a Customs and Border Protection app to schedule an appointment with a border officer to request asylum.

But for the most part, the order suspends longtime guarantees that give anyone who steps onto U.S. soil the right to seek a safe haven.

The executive action mirrors the legislation that Republicans blocked in February, saying it was not strong enough. Many of them, egged on by Mr. Trump, were loath to give Mr. Biden a legislative victory in an election year.

“Donald Trump begged them to vote ‘no’ because he was worried that more border enforcement would hurt him politically,” Andrew Bates, a White House spokesman, said in a statement on Tuesday. He added: “The American people want bipartisan solutions to border security — not cynical politics.”

Immigration advocates and some progressive Democrats have expressed concern that Mr. Biden was abandoning his promise to rebuild the asylum system.

“By reviving Trump’s asylum ban, President Biden has undermined American values and abandoned our nation’s obligations to provide people fleeing persecution, violence, and authoritarianism with an opportunity to seek refuge in the U.S.,” said Senator Alex Padilla, Democrat of California.

Mr. Biden said that those who believe his latest restrictions are too strict should be “patient.” He said in the coming weeks he would speak about “how we can make our immigration system more fair and more just.”

Tuesday’s decision is a stark turnaround for Mr. Biden, who came into office attacking Mr. Trump for his efforts to restrict asylum. During a 2019 debate, Mr. Biden, then a candidate running against Mr. Trump for the first time, excoriated his rival’s policies.

“This is the first president in the history of the United States of America that anybody seeking asylum has to do it in another country,” Mr. Biden said at the time.

Mr. Trump tried several times to close the U.S. border to asylum seekers, succeeding only in 2020 when he used a Covid-era emergency rule to seal the border to most migrants.

Immigration has proved to be a huge political vulnerability for Mr. Biden, reaching a crisis in December, when about 10,000 people a day were making their way into the United States.

Biden administration officials, panicked over those numbers, pressed Mexico to do more to curb migration. Mexican officials have since used charter flights and buses to move migrants deeper south and away from the United States.

The number of people crossing has plunged since then, though the numbers are still historically high. On Sunday, more than 3,500 people crossed without authorization, in line with the trends of recent weeks, according to a person with knowledge of the data.

Even with the executive order in place, migrants could still apply for other protections designed for those who can prove they will be tortured in their home country. But that screening has a much higher bar than asylum and as a result, administration officials said they do not expect many migrants to be screened into the United States.

People who cross illegally and do not qualify for those other protections would be subject to a five-year prohibition on entering the United States.

White House officials believe the order provides Mr. Biden an opportunity to take Republicans to task for dooming the bipartisan bill. That legislation also would have provided billions to the Homeland Security Department for more border officers and immigration judges.

Mr. Biden cannot provide those resources through executive action. White House officials for weeks said they preferred legislation over presidential proclamation because it would be more lasting and less exposed to a court challenge.

The order also comes with some political risks. Republicans have questioned why Mr. Biden did not take unilateral action at the border sooner. In January, he told reporters that he had “done all I can do” at the border and that he needed help from Congress.

“It’s all about show, because he knows we have a debate coming up in three weeks,” Mr. Trump said Tuesday on social media.

As Mr. Biden considered whether to take executive action in recent months, his administration has taken smaller steps to try to control those backlogs.

In May, the administration proposed a rule change that would allow officers to quickly identify people who are ineligible for asylum, such as those who have been convicted of serious crimes. Currently, they may be allowed to enter the country and wait months, or often years, for asylum proceedings. The proposal must go through a 30-day public comment period.

The U.S. Citizenship and Immigration Services also issued a new policy in May instructing asylum officers to consider whether applicants could find refuge in their own countries before coming to the United States.