Vice President Kamala Harris on Thursday acknowledged that President Biden got off to a “slow start” in the first presidential debate but insisted he ended on a strong note.

The remarks from the vice president, who would seem to be the natural successor should Mr. Biden step aside now, came amid a torrent of criticism from fellow Democrats about his performance and reports that party insiders want to dump him.

“Yes, there was a slow start, but it was a strong finish,” she said in a contentious CNN interview with Anderson Cooper. “What became very clear through the course of the night is that Joe Biden is fighting on behalf of the American people on substance, on policy, on performance. Joe Biden is extraordinarily strong.”

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Mr. Cooper pushed back saying the president’s performance was “disappointing” to his fellow Democrats. He pressed her to answer whether Mr. Biden who appeared weak and meandering on the debate stage was the same person she sees every day at the White House.

Ms. Harris did not defend Mr. Biden’s debate performance.

Instead, she urged voters to evaluate the president’s performance during his time in office rather than Thursday’s debate.

“I’m not going to spend all night with you talking about the last 90 minutes when I’ve been watching the last 3½ years of performance,” she snapped back.

Ms. Harris acknowledged that she couldn’t dispute that Mr. Biden got off to a “slow start,” but said the stakes were too high to focus on his debate performance.

“Do we want to look at what November will bring and go on a course for America that is about a destruction of democracy, electing a man who said he would be a dictator on Day One, or do we continue on a course that’s about strengthening America’s economy?” she asked.

In a more friendly interview with MSNBC host Rachel Maddow following the CNN appearance, Ms. Harris briefly reiterated her line about Mr. Biden’s “slow start” and “strong finish” before launching into a contrast between the candidates’ approach to answering the debate questions.

“What we had in Joe Biden was someone who wanted to have a debate based on facts, based on truth,” the vice president said. “And in Donald Trump, we have what we have come to expect, which is someone who’s going to push lies and distract from the reality of the damage he has created and continues to create in our country.”

Ms. Maddow did not ask further questions about Mr. Biden’s performance, instead allowing Ms. Harris to spend several minutes calling Mr. Trump out for lies he told throughout the debate.

Mr. Trump and his GOP allies, Ms. Harris argued, are attempting to “create fear based on fiction for the sake of getting and hitting political points.”

“What we know — and what he is acutely aware of — is what he has done and intended to do is not politically popular with a lot of people in both so-called red states and blue states,” she said.

Ms. Harris ended the MSNBC interview reiterating her support for Mr. Biden and jabbing at Mr. Trump for losing the support of former Vice President Mike Pence.

“Of the two people on that debate stage, only one of them has the endorsement of his vice president,” she said. “And let’s not forget that.”