The person most helped by Ms. Clinton's victory in Pennsylvania is John McCain. Clinton can continue to beat up Obama, who can continue to look like an effete wimp for getting beat up by a woman. Then, when Obama wins (as he ultimately will), he will no longer be seen as a visionary leader but as a guy who couldn't deliver the knockout punch despite a huge lead and who descended with Clinton into petty political mudslinging. His message of "Hope" and change is probably irreparably tarred.
Meanwhile, McCain is currently shoring up the middle-ground by distancing himself from all of Bush's myriad policy failures and trying to win over the moderate undecideds who usually decide these elections. And, after the conventions, the Republican Party's political machine -- which is even better at mud-slinging than the Democrats' and can still be quite effective despite Bush's nosedive in popularity -- will deliver an even more ruthless beating to Obama for being a left-wing liberal (which he is) who is going to raise everyone's taxes to fund a government bureaucracy that, if the Bush presidency has proven anything, is largely ineffectual.
Obama's best chance against McCain was to go into the general election viewed as a larger-than-life, Kennedy-esque icon for change, a beacon who inspires voters. That is the only way he could win the moderate voters who otherwise would gravitate toward the more moderate McCain. Much tougher to do that now, after what Ms. Clinton's campaign has done to him. It also doesn't help that he hasn't backed up most of his lofty ideals with any concrete policy proposals. He now looks like just another politician, and not a particularly strong one, at that. The longer this drags out, the better for McCain. The disaster for the Democrats is if Clinton wins the nomination by carrying the superdelegates despite having fewer overall votes, which will make the Democrats look just as bad as the Republicans looked after they (according to Democrats) "stole" the 2000 election. Plus, Clinton has no shot of beating McCain. As I noted in an earlier post, too many people hate her.