[mlo]

"Plain and simple. If President Biden, the presumptive Democrat Party nominee is not fit for reelection and is not fit to be prosecuted by Hur, he is not fit to be president and the party, his vice-president and cabinet need to invoke the 25th Amendment."

That's an argument, but it's not a legal argument. There's nothing in the law that says Biden must stay in the race if he stays as President.

[Yulee]

There's nothing in the law that says Biden must stay in the race if he stays as President.
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This seems to be, according to some sources, what will happen.

Then once a nominee is selected by the Democrats, the 25th Amendment will be initiated in order to place Kamala Harris in office, as the “First Black Female President” in history.

This being said, I would not put it beyond them to nominate Kamala Harris as the Democrat candidate for President. They then can run her as a sitting President for office.