What will Twitter do when the President loses the election, as he is likely to, and then immediately takes to Twitter to proclaim that a fraud has been perpetrated, as he is virtually certain to do in that scenario?

Will they really just tag his claims with a "this is misleading?" Or will they take the approach that is most certain to preserve order, and ban the President?

This is not a drill. This is a question that Twitter will be confronted with this once, and never again. No new policy need be created. If principles are compromised in doing this, it is a one-off.

Putting aside debates about cancel culture and ideological bias, Twitter already censors various kinds of uncontroversially harmful speech, and the President's claim--to the small but not insignificant segment of his supporters who are angry, credulous, and well-armed--that the election was stolen will surely qualify as such a statement.