Fight for reachable pragmatists, not ideologists
My guess is that Obama and the Democrats have largely written off the votes of the radical ideological-purist left. That group is, after all, at risk to sit out the election or vote third-party (and in some cases has explicitly threatened to do so) if the Democrats don't deliver on various litmus-test issues, some of which are neither practical nor possible -- and is truculently unmoved by the fact that the Republicans whom they would thus empower are far, far worse.
Democrats would be wise to focus instead on the working-class activism energized by the union-busting antics of Scott Walker and other teabagger kinglets. For these voters, having a decent (or at least not malignant) government is a matter of economic survival, not just some ideological abstraction. They know that in the real world, you need to choose the better of the available alternatives, not compile a checklist which the better party must satisfy to be worthy of support no matter how bad the other side is.
The elderly, too, despite the cultural conservatism of many of them, may well be reachable once they realize how profoundly the Republicans threaten Medicare and Social Security. And with their numbers and established habit of high turn-out for elections, they are a formidable bloc.
Another group with strong pragmatic reasons to fight for the Democrats is the gay community. The Republicans, since their 2010 electoral gains, have launched a sustained and vicious attack on gays' recent gains, pushing to re-instate DADT and fighting all- out against gay marriage with a campaign of nakedly-bigoted lies. The rabid fundamentalists who make up the present Republican base demonize gays -- almost literally in some cases. Gays would have more to lose than any other group if Republicans won more power next year -- and the community is willing to commit real resources to the struggle.
The course for Democrats is clear: go to the mat for worker and union rights, for the social safety net the elderly depend on, and for the civil rights of the besieged but tough LGBT community. Fight for the votes of these pragmatic, reachable constituencies. And for goodness sake, stop being shy about raising tax rates on the wealthy, which are now at historic lows. Americans across the board overwhelmingly support this. And Obama isn't the only one who can exert real power and take such initiatives. Congressional Democrats can and should, too.
And who knows? If the Democrats go all out in those struggles and win, some of the ideological purists may come home by November as well.