As I see it there are two different victory scenarios for McCain that I find plausible.
But first we should reconize the swing states that he absolutely must win in November.
No must-wins here!
Florida--27 electoral votes, obviously of immense importance. Fortunately, McCain appears to be leading here already.
North Carolina--15 electoral votes I thought this state was solid GOP but current polls show Obama within striking distance.
Virginia--13 Electoral votes, possible the most vulnerable Southern must-win state.
Misssouri--11 electoral votes, usually votes for the winner but has been leaning Republican recently.
Mississippi--6 electoral votes. Obama can win it if he drives the black vote through the roof but its unlikely to happen for reasons stated earlier.
Right now, McCain has a comfortable lead in the remaining Southern States.
Indiana--11 Electoral votes, again, supposed to be solid GOP but polls are tied.
Ohio--20 Electoral votes, the most vulnerable must win state considering that Bush won it by 2.1% in 2004.
McCain has confortable leads here, except possibly Nebraska's first Congressional district.
McCain is virtually assured to win traditional GOP strongholds like Wyoming, Idaho, Utah, and Arizona.
The first scenario is what I call the "Bush scenario". This envisions McCain playing defense and trying to win all 29 states that Bush won in both 2000 and 2004, limiting resources in other states. That would give McCain 274 electoral votes, four more than necessary, so it is technically possible that he could lose a small state like Montana and a congressional district in Nebraska as well and still win 270-168.
A drawback to this stategy, of course, is that Obama has made strong showings in Western swing states like Nevada, Colorado, and New Mexico and exerts a special appeal due in large part to his untraditional "new politics" that is attracting libertarian whites who would have been turned off by Clinton.
A bold alternative strategy could be to concentrate resources by trying to flip a midwestern/eastern state while writing off most of the west. For example, assume that Howard Dean's wildest dreams come true and Obama not only carries New Mexico, Colorado, and Nevada but also Alaska, Montana, and a Nebraskan district. But assume he loses the above states plus Michigan.
He still loses 168-270.
It may sound foolish to "write off" the west, but McCain may have to prioritize his resources if he is going to be massively outspent as many are suggesting.