Even if only 10% of Christians would not vote for him because he was Jewish alone that is a disastrous percentage in a national election.
Feingold would have to have a lot going for him to overcome that. Opposition to the war isn't enough.
But lets look at actual empirical data. Feingold is from a purple, heartland state. A state which elects both democrats and republicans. And his being a jew hasn't hurt him. I'd say Wisconsin is a good bellweather, for the "average" american state.
Now, Fiengold might get clobbered in the deep south. But, nearly any democrat would.