The Democrats are playing to gain a seat in the Senate. It costs them nothing to throw Conyers and Franken under the bus, because those two will be replaced by people who vote exactly like them. If, however, with their hasty show of righteous indignation less than a week before the election, they can swing the Alabama election against Moore, they gain a seat in the Senate.
The Republicans, for their part, will try to get Moore elected, and then toss him out after an ethics probe, to be replaced by another Republican. Stalemate in that case.