President Biden issued a dare to high-ranking lawmakers who were calling for him to step aside, and he told congressional Democrats in a letter that he was “firmly committed to staying in the race.”

Calling into “Morning Joe” on MSNBC today, Biden said he didn’t care about any of the “big names” urging him to drop out of the race. “If any of these guys don’t think I should run, run against me,” he said. “Go ahead, announce for president. Challenge me at the convention.”

This could be the most crucial week of Biden’s presidency, as he is facing calls to drop out from his own party. He is hosting the three-day 75th NATO summit in Washington beginning tomorrow, and he has promised a solo news conference at the end of the week. His performance will be scrutinized by Democrats to assess whether he can handle the off-the-cuff pressure that he struggled with during his debate with Donald Trump.

Biden also made his most formal entreaty to top donors since the debate, in a 19-minute call today. The president repeated that he was staying in the race and said they needed to shift the focus of the campaign away from him and onto Trump.

Related: A Parkinson’s disease expert visited the White House eight times in eight months from last summer through this spring, including at least once for a meeting with Biden’s physician. It was unclear whether he was there to consult specifically about the president or for unrelated meetings. The administration has said that Biden has no signs of the disease.