In 1992, the Republican excuse for obstructionism was that Bill Clinton didn't win a majority of the popular vote. (I still remember Bob Dole gleefully declaring himself the "representative of the 53% of the nation that voted against Clinton.")
In 2000, the Republican rallying cry against obstructionism was that the popular vote didn't matter, and we needed to heal the country and stand behind "our President."
Let's say Obama wins with 60% of the vote. What will they say then? I humbly suggest this: "In a year like this, he should have won 70% of the vote, therefore he's not legitimate." Start your engines!
And my prediction, if anyone cares: Obama, 278 electoral votes. (It's low, compared to some, because electronic voting favors the Republicans.)