WASHINGTON — After he gave his State of the Union speech in March, President Joe Biden seemed to have quieted persistent fears that, at 81, he was no longer up to the job.

He spoke forcefully and jousted with Republican lawmakers who had jeered his message. But a Democratic lawmaker who shook hands with Biden in the House chamber that night was troubled by his appearance.

Biden, the congressman said in a recent interview, looked “frail and weak.”

Biden’s poor showing at the debate with Donald Trump last week threatens to end his campaign just four months before the election. Hoping to salvage his bid for another term, he is asking voters to weigh the 90-minute debacle against what he says is a 3½-year record of achievement in office.

Yet the notion that last week’s debate was an anomaly doesn’t jibe with the impressions of some Democratic lawmakers who’ve seen him up close and come away doubting his capacity to hold office.

Far from a one-off, the debate revealed the same worrying traits — memory lapses, incoherence, a vacant look — these officials say they’ve noticed in Biden’s company throughout his term.

“The country saw [at the debate] what those of us who have had personal interactions with him have all known for the last 2½ years,” a senator said, speaking on condition of anonymity to discuss Biden’s fitness.

A large, well-funded team of aides surrounds Biden and has worked to shield him from the embarrassments that might befall a president of advanced age. He often boards Air Force One through a shorter staircase leading to the belly of the plane to minimize the risk of a fall. No president other since Ronald Reagan has given fewer interviews, a forum in which he must nimbly field questions without the benefit of a script.

The debate stripped away the protective cloak. Now, Biden faces a crisis in which he needs to demonstrate — quickly — that he can function at the level the presidency demands. In a phone call this week with a close ally, Sen. Chris Coons, D-Del., Biden affirmed that “he’s got to do some things to win back the confidence of the American people,” Coons said in an interview.

The two men also discussed ways to overcome the fallout from the debate. One problem Biden acknowledged was that he came prepared to cite facts and figures that don’t carry much weight in a match-up with Trump, Coons said.

“Donald Trump is a very different kind of opponent,” Coons said. “Donald Trump stood there and unleashed a torrent of angry invective that Joe Biden could still hear even though the mics were off.”

Coons, who once worked for Biden as a senatorial intern, said he remains confident in his cognitive abilities. In the phone call with Biden, he said, the president was “sharp, engaged and energetic.”

“I’ve never had a moment that led me to question his mental acuity,” Coons added.

Others see a more diminished figure. A third Democratic lawmaker said that in recent months, Biden looked “exceedingly tired” when the two were together. In other dealings this year, Biden seemed “far from optimal.” Though the debate was the “worst” version of Biden, the lawmaker said, “it was not an aberration.”

A group of about 40 Democratic lawmakers have been texting one another since the debate, this House member said, and not one of them believes Biden should stay in the race.

In reply, a Biden campaign spokesperson cited various members of Congress who’ve vouched for Biden’s acuity.

Rep. Gerry Connolly, D-Va., a former Biden staff member from Biden’s time in the Senate, said, “I will tell you that in my encounters with Joe Biden as president, I have not experienced what I saw last week” at the debate.

“The minute I see him, he knows me by name,” Connolly continued. “He will bring up mutual friends and colleagues that worked with us in that time period by name and is genuinely interested in how they’re doing.”

“I have certainly witnessed the aging of my former boss, sure, but not mental decline,” Connolly said.

People grow old in different ways, and the presidency tends to act as an aging accelerant. In 1990, a year after having left office, Reagan testified at the trial of a former White House official. He was 79 at the time, two years younger than Biden is now.

Reagan didn’t remember the name of his former chairman of the Joint Chiefs of Staff. He used the words “I don’t remember,” or some equivalent, at least 124 times, according to a Los Angeles Times account of his testimony.

There are plenty of occasions when Biden demonstrates impressive command of his faculties, aides say.

While he was prepping for a major speech in France last month for the 80th anniversary of D-Day, Biden took pains to ensure the text did justice to the Army Rangers who scaled the cliffs in the face of German machine gun fire, a senior White House official said.

An early draft said the soldiers were thinking about preserving democracy as they ascended rope ladders at Omaha Beach.

That didn’t sound right to Biden, the official said. After he reviewed the draft in his Paris hotel, Biden questioned an aide about whether the future of democracy is something combat soldiers would really be discussing while they were evading enemy fire.

No, it’s not, the aide replied.

Biden insisted that the reference be scrapped in favor of something more genuine.

The speech he delivered at Pointe du Hoc instead recounted an exchange between two soldiers, one of whom said, “I’m not sure I can make it,” and another who yelled back, “You’ve got to hold on.”

It’s far from certain that Biden can hold on. He is placing a big bet on an interview scheduled Friday with ABC News, seeing it as a chance to quell the widespread doubts that have arisen since the debate.

“One of his core challenges is to show people that this was a moment,” Coons said. “This was a bad debate night for a range of reasons — not some concealed condition. And we talked about the things he needs to do to build public confidence in that.”

Yet the interview comes eight days after Biden’s appearance onstage with Trump — time enough for perceptions of his poor performance to calcify.

Even if the interview goes well, it may not be enough to rescue his candidacy.

“There is a great deal of love for this man and his family. And there is great deal of bewilderment among us about why he, and the people around him who presumably love him, are letting him take his historical legacy and throw it down the drain,” the third Democratic lawmaker said.

“If he loses to Trump after that debate, that will be on his tombstone instead of his achievements. It’s an absolutely legacy-defining moment,” this lawmaker added.