Well first of all, this is the wrong forum.
But the answer is minorities and/or young voters. But that analysis is a bit too simplistic. Political environment matters too. It's not like 2010 would've suddenly been a decent year for Democrats if Obama was on the ballot (he likely would've lost even with the increased turnout). Turnout problems certainly hurt the Democrats in midterms, but it's not the only explanation for the results of the past 3 elections.
This, as well as the fact that Independents swung heavily against Democrats in 2010. Base turnout is important, but if the rest of the electorate is drive to vote against you, the best you can do is hope to minimize your losses.
