President Biden on Thursday answered questions from reporters about foreign policy, whether he is up to defeating former President Donald J. Trump and why he is resisting calls from Democrats to end his candidacy, as he sought to recover from a crisis of confidence that has engulfed his campaign.

With a growing number of Democratic lawmakers, donors and elected officials urging Mr. Biden to drop out of the race after a disastrous debate performance last month, the closing news conference of a NATO summit in Washington became a high-stakes chance for the president to quiet concerns about his candidacy. The results were mixed.

Mr. Biden stumbled early but remained defiant in the face of questions about his fitness to continue his campaign. He struggled to articulate a cohesive case for his candidacy, even as he gave a forceful defense of his record and showed a strong command over foreign policy.

Here are five takeaways:

He said he is not leaving.

Mr. Biden vowed to stay in the presidential race. “I’m determined on running,” Mr. Biden said. He dismissed polling showing him losing to Mr. Trump and insisted, “I think I’m the best qualified person to do the job.” But he also acknowledged that the schedule of the presidency had become challenging. “I just got to, just, pace myself a little more,” Mr. Biden said.

Before the debate, he said, his schedule was “full-bore” and he made the “stupid mistake” of too much travel and too many late nights before the debate. Mr. Biden also blamed his staff for the packed days. “I love my staff,” Mr. Biden said. “But they add things. They add things all the time at the very end.”