Representative Nancy Pelosi, Democrat of California and the former speaker, said on Tuesday that it was “legitimate” to ask whether President Biden’s bad night at the debate was a one-time flub or “a condition,” adding that former President Donald J. Trump should have to answer the same question about the falsehoods he uttered.

“I think it’s a legitimate question to say is this an episode or is this a condition,” Ms. Pelosi said in her first extended remarks since Mr. Biden’s halting and disjointed performance at the debate.

“When people ask that question, it’s legitimate — of both candidates,” she added. “What we saw on the other side was lying.”

Ms. Pelosi heaped praise on Mr. Biden, saying he was “masterful in helping to write and to pass” a slew of legislation that Democrats passed early in his presidency when they had control of Congress. “He has a vision,” she said. “He has knowledge. He has judgment. He has a strategic thinking and the rest.”

But she conceded that she was hearing “mixed” feedback from Democratic donors on whether the president was up for running for another term in office, and said that some in the party argue that the stakes are so high that Democrats should not risk losing with a candidate who isn’t up to the task.

“Some people are saying because Donald Trump is such an authoritarian and autocrat, we have to win this election,” she said. “This is not a normal election where you want to win, if you don’t, you cooperate and do the best you can for the country and hope to win the next time. This is something that is undermining our democracy. He must be stopped. He cannot be president. Therefore, people are very concerned.”