With top Democrats expressing alarm over President Biden’s shaky debate performance Thursday night, Vice President Kamala Harris defended her boss in interviews on CNN and MSNBC, arguing that Mr. Biden should be judged on his record in office rather than the moments on the stage where he faltered.

“Joe Biden is extraordinarily strong,” Ms. Harris said, as Anderson Cooper of CNN repeatedly pressed her to assess how Mr. Biden handled the evening.

She conceded that he did not perform expertly at the start of the debate.

“It was a slow start, that’s obvious to everyone,” Ms. Harris said. “I’m not going to debate that.”

At a virtual debate watch party with supporters before her CNN appearance, Ms. Harris appeared to read from prepared remarks to assure supporters.

“He got into a groove where it counted,” Ms. Harris said in her remarks. “Our president showed that he will win the election.”

On CNN, she argued that the election must be decided “on substance,” not on debate style. And she sought to highlight the false claims made by former President Donald J. Trump throughout the debate and raise alarm about how he might restrict abortion access if he returns to office.

In the MSNBC interview that followed, she repeatedly described Mr. Biden as “clear” in his messaging and said that, during the debate in particular, his pitch to enshrine abortion access in a second term was firm.

Ms. Harris conceded again that Mr. Biden had a slow start, but added that “I thought it was a strong finish.”

Ms. Harris is expected to address supporters at a rally in Las Vegas on Friday.