Bayh has always supported the war.
He has proposed a bill to end the "Patriot Penalty" so that soliders won't have their pay cut once they come back from the war.
Pro-war is a definite negative when it comes to the primary. And if they don't admit it's a mistake, it will hurt them in the general election too. Kerry did ok since he admitted it was a mistake, but it would have been better if he hadn't voted for the war in the first place.
Had Kerry voted against the war, he would have lost in a landslide. The GOP would have had far more success linking him to his antiwar past had he not voted for the Iraq War. He played it as well as he could. He votes for it, then says that it was handled badly, thus making it a mistake.
By the way, the candidate that didn't claim that it was a mistake won the election.