Biden Says Harris Is Qualified to Be President, but Vows to Remain in Race

President Biden said on Thursday that Vice President Kamala Harris was qualified to be president, but pitched himself as the best person to defeat former President Donald J. Trump in November.

During a news conference, Mr. Biden defended Ms. Harris’s readiness to take charge amid reports that he feared she could not beat Mr. Trump even as he made clear that he would not step aside. But in his answer to a question about Ms. Harris’s chances of beating Mr. Trump, he confused the two leaders’ names, committing just the sort of verbal misfire that has unnerved Democrats and prompted them to wonder if she should instead be the nominee.

“Look, I wouldn’t have picked Vice President Trump to be vice president, did I think she’s not qualified to be president,” Mr. Biden said. “So let’s start there, No. 1. The fact is that the consideration is that I think I’m the most qualified person to run for president. I beat him once, and I will beat him again.”

Polls in recent weeks have generally showed that Ms. Harris is just as likely, or possibly more likely, to defeat Mr. Trump, and the former president’s campaign has ramped up attacks on her recently.

Following Mr. Biden’s disastrous debate performance two weeks ago, panicked donors, political groups and even allies began to rally around Ms. Harris as a potential placement for him. Ms. Harris has stood behind Mr. Biden’s decision to stay in the race.

The Biden campaign has been quietly testing Ms. Harris’s chances against Mr. Trump this week, with some speculation that the results would determine whether Mr. Biden would consider dropping out of the race.

Asked whether he would reconsider his decision to stay in the race if his team showed him data that Ms. Harris would fare better in a