Gravel would no chance. First of all, remember his 2008 bid when he was destroyed by media freeze. Second of all, he switched to LP. Third of all he'll be 82.
Gravel was my favorite in 2008 primaries even when he stood no chances.
And no one would primary Obama. If Kennedy couldnt' primary Carter in 1980, why do you think someone can primary incumnet at all?
Kennedy almost certainly would have beaten Carter had Carter not been boosted at just the right time by the emergence of the hostate situation in December 1979.
