Matt Yglesias cites the media's utter lack of interest in John McCain's courtship of, and endorsement by, anti-Catholic, anti-semitic John Hagee as a concrete example of how the ongoing Democratic race benefits John McCain:
I've been Hagee-bashing since before it was cool, so pisses me off, too. But realistically it's not the press and the cable networks that gave McCain a pass, it was Hillary Clinton and Barack Obama. They gave him a pass because, of course, they were arguing with each other. For a little while during the Wisconsin-Texas interregnum, Obama did pivot in the direction of McCain and it gave Clinton the opportunity to smack him over the head with a frying pan. I assume neither campaign is going to make that mistake again until this thing is actually wrapped up. But that means that there'll be nobody effectively pressing the media with anti-McCain talking points. It also means that Clinton will continue re-enforcing whatever good lines of attack McCain comes up with against Obama, and if McCain starts delivering good anti-Clinton lines, Obama will probably start re-enforcing those, too.
This kind of dynamic hardly guarantees defeat in November, but it's hard to see how it helps.
It's not just the Hagee episode. This is also a big part of the reason that McCain's attack on Obama for contemplating breaking his public-financing "pledge" got so much more attention than McCain's own, possibly illegal (and clearly far outside the spirit of the law) decision to opt out of public financing for the primary after having opted in last year. Had there been a Democratic nominee to forcefully make the case that McCain was being disingenuous, the media would have given this far more attention. But there wasn't, so it didn't.