He knows Biden is likely to fold under pressure.
The Democrats now have an unhappy choice:
Either continue to hide Biden from the public, and risk an epic Trump landslide, as voters consider hiding in the basement to be a virtual concession of the race to President Trump.
Or feed Biden to the lions - trot him out in front of the cameras and let him debate - and risk dementia related blunders that could also lead to a Trump landslide.
I think with the latter alternative, at least there is a chance Biden may not embarrass himself at the debates by forgetting where he is, taking his pants off and folding them neatly over the podium...
...or confusing the President for his old arch nemesis Corn Pop, and going after him with a raised shoe...
...or removing his dentures on prime time TV and placing them in his water glass...
..or accidentally answering one of the debate questions truthfully...