This NPR piece about McCain's vulnerabilities going into the general election is succinct and informative, and it basically boils down to one thing: McCain is a Republican. He's forced to move to the right to shore up his base, which is not nearly as enthusiastic about him as the Democrats are about Obama, and he's forced to move back to the center to court the independents he desperately needs because his base has shrunk dramatically thanks to general disgust with President Bush and years of Republican domination. McCain must walk a very fine tightrope to even have hope against Obama in November, but with the general election concluded Obama, the DNC and the 527s have a ton of money to spend pointing out to voters time and time again that McCain publicly argues for continuing to carry out some of Bush's most atrocious policies, like the war in Iraq, tax cuts and now efforts to drill for oil in protected areas. But as polls are appearing to show, even the best run campaign may not be enough against Obama, who thanks to the end of the Democratic primary is now pulling ahead in crucial swing states. Without the distraction of the Democratic primary Obama is free to hit McCain on these issues, and voters are free to see exactly where McCain stands.
UPDATE: David Weigel at Hit & Run has more on exactly how difficult of a road McCain has ahead of him and how quickly Obama is catching up/racing ahead in the polls.