But even more, Biden is worried about losing the energy of his base. It is still a tight race in the states where the election will be decided. The last thing Biden can afford is a revolt from the Bernie Bros. and AOC’s “squad.” He knows Hillary Clinton lost the 2016 election in its last couple of weeks, mainly by taking the Democratic base for granted. He is not going to repeat that mistake. (And, of course, no FBI director is going to announce the reopening of a criminal investigation against him just days before voters go to the polls — so he’s got that going for him.)

Beyond that, Biden realizes that Republicans have the votes to confirm Judge Amy Coney Barrett, President Trump’s impressive Supreme Court nominee. She is apt to keep her cool and perform well through what promise to be contentious confirmation hearings. Biden and other party leaders have calculated that keeping alive the threat that Democrats may retaliate by packing the Court is their sole thin hope of peeling off enough Republicans to stop Barrett’s appointment. Of course, it would be foolish for Republicans to cave: They know Democrats were threatening to pack the Court long before there was a vacancy for Barrett to fill — it was all the rage during Justice Brett Kavanaugh’s confirmation debacle — and, like Biden, they also know that Democrats will not have the votes to end the filibuster.

In the main, Biden has been refusing to answer the court-packing question because he is trying to keep his voters on-board and excited at the prospect of big changes.