Aside from being America’s oldest president, one with failing mental acuity, 
Joe Biden is also the most unpopular president ever. 
Over seventy percent of Americans do not want him to run for reelection.
Many prominent Democrats are working to push Biden out of the race.
If they succeed, Kamala Harris is the most likely choice to succeed him. 
But there are others, including Gavin Newsom.

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Kamala Harris

Gavin Newsom