Because he's smarter and more competent than Santorum or Gingrich, and this is clear to a large part of the electorate, especially moderates. I don't know if he'd do better than Bush against Gore (I chose those two because that election was a statistical tie, so if he does better, he wins; if not, he loses). Obama has a decent chance of winning of course.
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Congratulations to SuperNiceDog, Guru Winner, who was smart enough to pick
your 7 time champion, Link.