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Andrew Neil seems to have explained it right.

"If Biden not fit to run again he’s not fit to remain as President . Not really. Now he’s off a gruelling campaign schedule his limited energies can be devoted to being President and he has a strong team around him. The country will be safe thru January.
It’s all been orchestrated by Obama and his surrogates. So what?
It’s just campaign noise/bluster from both sides. All that matters is that barely a week ago Trump pretty much had it in the bag. Now it’s a close contest. A contest which is barely underway and whose outcome is now impossible to predict.
We will soon see how long the Harris Honeymoon lasts and if she is a better campaigner than she has been in the past.
And if Trump still has the knack of being his own worst enemy."