If this goes to court, I would hate to be the Dem counsel trying to argue that the nominated incumbent decided not to run because the polls told him he was losing!! I doubt this would be an issue if he were 14 points up on Forrester, so the reasons for quitting are specious. It would seem to me that in accepting a nomination, you are pledging to the people that voted for you and your party that you will see the election through, win or lose.
Of course, we are talking about a Democrat, and especially corrupt one at that, so what does a pledge mean anyway? The Dems just get more loathsome every election.