I'm basing this claim on the current data from the ever-excellent 538, plus a few other observations about the campaign.
538's most recent projection of the national vote share is Obama 51.8, McCain 46.8, or a +5 popular vote margin. I then compare with the current state-by-state projections to see what would happen if we see a uniform 5 percent swing to McCain, to make the popular vote exactly tied.
The result: Obama still wins the electoral college comfortably, picking up all the Kerry states + IA, NM, CO and VA, for an EV total of 286.
If McCain wins the popular vote by 0.4% he would pick up Colorado, but still lose the election 277-261.
Only if he gains a popular vote win of >1.2% would he pick up NM and VA and win the election.
However, there is good reason to believe that any swing towards McCain would NOT be uniform, and that Obama would do significantly better than McCain in the key battleground states. The reasons: money and organisation.
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