It's the inevitable sequelae of running a 'hate Bush' campaign. The positive is that you have a massive built-in voting block. The negative is that you are then immobilized by the nature of the coalition you've assembled.
How do you campaign to both the anti-war Bush haters, and pro-war Bush haters?
How do you campaign to both the pro-gun control Bush haters and the 2nd amendment loving Bush haters?
How do you campaign that Bush spends too much before a progressive crowd and a fiscally conservative crowd?
So you center on the single uniting characteristic, Bush sucks. But after a while, fatigue sets in and some say "well he sucks, but not THAT much, sheesh, lay off for a sec willya."
Then those seeking to help you either go wildly over the line in their claims, or outright manufacture 'evidence' in support, and people start saying "Bush may suck, but the way these guys are lying to us, he might not suck as bad as we've been led to believe."
Now you're in a tight spot. That saw about being a war hero has tired some and soured others. The Bush sucks thing is taking heaping rotted scoops of flesh out of the hindquarters of a long dead horse. AND, discussing substantive issues is now a mindfield. Will I get more votes on the margins WITH a gun in my hand, or denouncing the sunset of the weapons ban? Will I get more votes on the margins by promising to get the troops home, or by promising a continued resolute war campaign?
And people see that vaccilation, and irrespective of their support or lack thereof based on your stance on an issue, they've now got that 'flip-flopper' thing reinforced BY YOU.
Kerry's only hope is a resurge in Bush hate. His fortunes now lie with the increasing dead in Iraq.