ANALYSIS — Democratic lawmakers are setting a high bar for what President Joe Biden needs to show voters as the party splinters over his cognitive abilities and candidacy.

But can the 81-year-old commander in chief clear it?

“Biden’s candidacy, today, does not feel sustainable,” former Rep. Charlie Dent, R-Pa., said in a Monday phone interview. “I’m not one to put a timeline on this, but boy, the narrative is not getting any better.

“I have to think, if you’re a Democratic congressman right now, down-ballot races matter. And congressional Democrats are concerned about maintaining their majority in the Senate, and they know their ability to take back the House is probably less likely today than it was before the debate,” Dent, a Biden voter in 2020, said of Biden’s poor June 27 performance against Donald Trump, the expected GOP nominee.

“An epic collapse at the top of the ticket is disastrous for down-ballot races. … The bottom line is: A lot of people have carried the president’s water, and now they feel gaslit or betrayed.”

Biden on Monday told MSNBC he still believes he is the best candidate to defeat Trump, though he called his poor debate performance a “terrible night.” He called his term a “significant run” and said his February physical exam included a neurological check-up that turned up no medical issues.

“It drives me nuts, people talking about this,” he added, listing his public appearances since the debate. “I’m not going to explain any more about what I should or shouldn’t do. I am running.”

The president was equally blunt in a letter to congressional Democrats sent Monday, writing: “I want you to know that despite all the speculation in the press and elsewhere, I am firmly committed to staying in this race, to running this race to the end, and to beating Donald Trump.” The president concluded the letter by telling Capitol Hill Democrats that “any weakening of resolve or lack of clarity about the task ahead only helps Trump and hurts us.”

Biden will get a number of chances to prove himself during what will be another make-or-break week as he hosts a NATO summit in Washington, D.C., that will culminate in a solo news conference that Democratic lawmakers and donors surely will be watching closely.

While the president had a dismal debate performance against Trump, Biden was more clear and showed more vigor during a Friday rally in Madison, Wis. Still, he was at times dismissive about Democrats’ concerns about his ability to defeat Trump — and, at other times, indecipherable.

Biden on Monday showed no signs of considering dropping out of the race after telling ABC News in an interview that aired Friday evening that only the “Lord Amighty” could drive him from his rematch with Trump.

But he appears to have lost even some of his closest allies and admirers on Capitol Hill.

Because lawmakers face a new group of reporters around almost every corner when they are in session, their suddenly bleak collective assessments of his mental capacity and odds of beating Trump will do little this week to change popular perceptions about the president.

Rep. Adam B. Schiff, D-Calif., and other lawmakers have gone so far as to call for Biden — and the 78-year-old Trump — to take cognitive tests so voters have a clear picture about each candidate’s true mental health.

“Look, I’d be happy if both the president and Donald Trump took cognitive tests,” Schiff told NBC’s “Meet the Press” on Sunday. “I think, frankly, a test would show Donald Trump has a serious illness of one kind or another.”

But Schiff, the Democratic nominee for a Senate seat in his state, also did not say that Biden should stay in the race.

“If [Biden’s] decision is to run, then run hard and beat that S.O.B.,” he said. “And if his decision is to pass the torch, then the president should do everything in his power to make that other candidate successful.”

Some longtime Biden allies such as Sen. Thomas R. Carper of Delaware are standing by him. “I met him 50 years ago and he’s been my friend for 50 years,” Carper told reporters Monday. “I’ve supported him in everything that he’s run for. … I have no interest in walking away from him today, or tomorrow or the day after that.”

Carper, a Democrat who is not seeking reelection, said Biden will have “plenty of opportunities to show if he still has what it takes, and I think he deserves a chance to demonstrate that.”

“The goal here is to reelect President Biden,” said Sen. Mark Kelly, D-Ariz. He expressed skepticism about polls showing Biden losing Kelly’s home state to Donald Trump. “What I can tell you from my personal experience with public polling, it’s not entirely accurate.”

Biden will not get any younger before Election Day. He has perhaps the most stressful job in the world. And he has suffered a number of tragedies in his personal life. He admits he is not as smooth a talker these days, nor as feisty of a debater. Even on Monday morning, he started some thoughts before pivoting to another, and slurred some words.

Sen. Martin Heinrich, D-N.M., said Monday that “President Biden needs to continue to demonstrate that his debate performance was just a bad night, and that he has a clear path to defeating Donald Trump,” adding in a statement: “Our democracy hangs in the balance.”

House Armed Services Committee ranking member Adam Smith, D-Wash., told CNN on Monday that he wants Biden to clear a path for Vice President Kamala Harris. “Personally, I think Kamala Harris would be a much better, stronger candidate,” he said.

‘Not going anywhere’

Sen. Brian Schatz, D-Hawaii, said Democratic lawmakers need to have a series of “family conversations” about the best way forward.

But Brad Bannon, a Democratic strategist, said Monday the president is not trying to out-swim the sharks in his own party.

“Nothing will satisfy the doubting Democratic dissidents. They want President Biden gone and they have set the bar so high that an Olympic high jumper couldn’t get over it,” he said in an email. “The president could recite the NATO charter in French while doing a handstand and it wouldn’t change any minds. The blood is in the water. … It will be tough for President Biden to hang on.”

Michael Bitzer, a political science professor at North Carolina’s Catawba College, said Monday, “Biden must engage in a reassuring campaign to his own base and to that small sliver of the persuadable electorate and give them a sense of a future of competence, reassurance, and forward looking. If it’s a negative partisanship campaign strategy, it feels like that’s advantage Trump, as he can rile up his base more effectively than Biden can.

“If this week’s solo press conference falls flat, or worse yet is a repeat of the debate debacle, then no amount of letters to congressional Democrats or interviews will help construct that reassurance campaign,” he added in an email.

For now, at least, Biden has remained defiant.

At several points in the Monday MSNBC telephone interview, Biden went hard against Trump, calling him a “liar” who is only out for himself.

“By the way, you hear Trump talking about all he’s done and all that. And you know, Trump, Trump at the debate night, he’s a pathological liar,” the president said Monday morning. “He lied about Roe, he refuses to accept the outcome of the [2020] election, he refused to condemn Jan. 6. And he says, they claimed, he spoke with [Russian President Vladimir] Putin before he invaded [Ukraine].

“What the hell are we doing, Joe?” Biden asked “Morning Joe” host Joe Scarborough, a former GOP House member. “What are we doing? This is bizarre.”

Biden took a direct shot at his critics, showing flashes of his “Scranton Joe” persona that was a more common feature during his decades in the Senate than his time as vice president and now president.

“I’m getting so frustrated by the elites … in the party. ‘Oh, they know so much more,’” he said mockingly before issuing a challenge: “Run against me. Go ahead, announce for president. Challenge me at the convention.”

While House and Senate Democrats returned to Washington on Monday to opine about Biden and his embattled reelection bid — with some expected to formally call on him to step aside — the president dug in further during his morning interview.

“The bottom line here is: We’re not going anywhere,” he said of his campaign. “I’m not going anywhere.”

Daniela Altimari and Aidan Quigley contributed to this report.