There are 2 ways that Obama can attack McCain that will ensure his victory:

1. Hillary can come in hard on women’s issues, reproductive rights, etc. McCain has very unpopular positions (at least they are extremely unpopular with the “PUMA”s) on these issues. He has specifically called for an overturn of Roe v Wade, for example. Tie that with healthcare, and put Senator Clinton out front, stressing the importance of those issues to women and that women cannot afford for McCain to win, and you get an Obama victory.
2. They can “Goldwater” him. The surrogates can start by pressing McCain on international issues, such as Pakistan, Iran and Georgia. He will eventually respond with something hawkish enough for the Obama campaign to use in an expensive television blitz to portray him as a dangerous extremist, the way LBJ did to Barry Goldwater. The flip side of being “strong on defense” is being overzealous, keeping the country in constant, debilitating warfare.