... but are you sure you have good reasons to make this assumption that such a big majority of Obama's votes in 2008 consists of liberal Democrats who want him to be more liberal than he is? American liberals, particularly those on the coasts, have a way of overestimating to what extent the Democratic party as a whole resembles them.

To be sure trzaska and Joel make a good point when they say low turnout could hurt Obama, but I dare say Obama might very well end up losing significant amounts of voters who voted for him last time and will vote Romney now (or he might not - it's too early to call the election, obviously), because they are centrists and they weren't impressed by his achievements these past three-odd years. Just because he didn't make his liberal base happy doesn't mean he made the center happy. And of course with the electoral college working as it does, he doesn't need to lose such large numbers to lose a few key swing states, and the election with it.

Potentially rather a lot of things, depending on the political stances of said former Obama supporters. Obviously a GOP candidate would be stealing votes from Obama's right flank, not his left - your argument seems to boil down to the notion that he doesn't have a right (or centrist, at any rate) flank and all his votes were solidly liberal ones, which seems like a fairly absurd position to me.