This is entirely speculative, of course. In fact, I think Hillary would be nuts to challenge Obama no matter how bad his numbers or how badly the Democrats are trounced in November.

Those very same people on hands and knees begging Hillary to run after the Democrats are spanked in November will be the first ones kowtowing to Obama when the going gets tough. Only someone with a supreme ego and a rock solid belief in their own righteousness would challenge a sitting president from their own party.

So maybe it's not such a farfetched idea after all?

Gallup:

If Secretary of State Hillary Clinton were to challenge President Barack Obama for the Democratic presidential nomination in 2012, she would currently have the support of 37% of Democrats nationally, while 52% would support Obama.

[...]

Presidents with relatively low job approval ratings heading into a possible re-election bid are vulnerable to intra-party challenges. As two examples, Gerald Ford and Jimmy Carter were challenged for their respective parties' nomination as sitting presidents -- Ford by Ronald Reagan, and Carter by Edward Kennedy and Jerry Brown. Ford's job approval rating had reached as low as 37% in 1975, the year before he ran for re-election, and Carter's had reached 28% in 1979. Obama's current weekly job approval rating is 44%, albeit with more than two years until the next presidential election.

Obama's strengths among Democrats in the hypothetical matchup with Clinton lie with college graduates and liberals, the latter of whom make up about 36% of this sample of 859 Democrats and Democratic-leaning independents. Clinton does better among less well-educated Democrats and bests Obama among conservatives, who make up 22% of Democrats. Clinton does slightly better among women than among men.