Accept!
Every Democrat, including Hillary Clinton, should support this and chip in. This is not going to get Sanders the Democratic nomination, Clinton should not fear that, and this gives Demos a free shot at Disco Donald. He has never debated a Democrat, obviously, and his Republican opponents did not know how to respond to his candidacy. Democrats don't have that problem! They can try out lines of attack for the Fall without concern for repercussions.
Oh God, do it Dems, just do it.