Biden takes questions about his mental and physical fitness

President Joe Biden vigorously defended his fitness for office in a high-stakes press conference Thursday evening, fielding several questions about his age and calls for him to step aside.

“I think I’m the most qualified person to run for president,” he said, adding that he believes he is the best candidate to beat former President Donald Trump in November. “I beat him once, and I will beat him again.”

Early in the press conference, in a moment that was immediately seized on by his opponent Donald Trump, he flubbed the name of Vice President Kamala Harris.

Biden defended himself on questions about his health and whether he could continue to work aggressively through another term.

Biden said he has taken three “significant and intense neurological exams,” most recently in February, and would take another one only if his doctors recommend it. “They say I’m in good shape,” he said.

Still, he lamented that “no one’s going to be satisfied” with whatever medical information he release, suggesting his opponents will find reasons to doubt it.

The press conference comes as the embattled president doggedly tries to assuage the public and his own party that after a disastrous debate performance, he has the mental and physical stamina to be commander in chief and defeat Trump.

In the two weeks since the debate, Biden has seen several members of his own party call on him to leave the race but Biden insists he isn’t going anywhere.

Follow live coverage and watch Biden’s press conference on NBCNews.com.

He chalked up his “stupid mistake” in last month’s debate to his “full bore” schedule and need to rest more.

“I’ve just got to pace myself a little bit more. That’s what it was about,” he said in response to a question about reports that he told allies he needed to shorten his schedule. “Where has Trump been? Riding around on his golf cart.”

He also cast some blame on his staff for over-scheduling him. “I love my staff but they add things. They add things all the time,” he said, before stopping himself. “I’m catching hell from my wife.”

Later, though, he acknowledged the demanding nature of the job.

“If I slow down, I can’t get the job done. That’s a sign that I shouldn’t be doing it. But there’s no indication of that yet, none,” he said.

He was asked if he worried about his legacy if he stayed in the race, resisting calls from fellow Democrats to step aside, and ends up losing to Trump.

“I’m not in this for my legacy. I’m in this to complete the job I started,” the president added.

Still, Biden made two gaffes Thursday evening alone that are likely to overshadow the message he wanted to send.

After opening remarks, read from a teleprompter, Biden’s first question was about whether he was confident Vice President Kamala Harris could replace him if necessary. Biden conflated her name with his opponent’s.

“Look, I wouldn’t have picked Vice President Trump to be Vice President, to — think she was not qualified to be president. Let’s start there,” Biden said in response to the first question.

Hours earlier, at the conclusion of a NATO summit in Washington, Biden appeared to confuse the leaders of Ukraine and Russia.

“Ladies and gentlemen, President Putin,” Biden said as he introduced Ukrainian President Volodymyr Zelenskyy at a on Thursday.

The president stepped away from the podium before turning back around and corrected himself. “President Putin? He’s gonna beat President Putin! President Zelenskyy.”

“I’m better [than Putin],” Zelenskyy joked as he shook Biden’s hand.

Biden has been the world’s leading champion of Ukraine throughout two years of bloody warfare to turn back an invasion from Russia, led by President Vladimir Putin.

At Biden’s last press conference, in February, where the president angrily denounced a special counsel report that called him “significantly limited,” Biden referred to Egyptian President Abdel Fattah el-Sissi as the “president of Mexico.”