Leeann Tweeden said that she went public when she heard Jackie Speier talk about being assaulted as an intern. I don't think Franken brought it up at all.
Franken did later say that he would refer himself to the ethics committee, but that was a last-ditch effort to protect himself. Franken knew that the Democrats would sacrifice him to gain leverage on opposing the seating of Roy Moore if he won.
The problem for Franken was that Democrats couldn't wait for a month until the Alabama special election to act.
-PJ
He’s a Democrat and could have gotten away with blowing it off as just a joke (which it actually was). He specifically chose to leave rather than fight an easily winnable fight - that’s a guy who was looking for an excuse to make an exit.
Bingo!!