In a word: Republicans.
It's really no surprise. The party in the White House almost always loses off-year elections. The president's approval ratings hover in the low 40s, solidly "meh" territory, and in some regions it is much lower. Enthusiasm among Democrats is low, while the energy level on the Republican side, especially among Tea Party conservatives, is high. The conditions are ripe for a Republican year.
I predict the Republicans will pick up enough seats to control the Senate, probably 52-53. Iowa, Alaska, Kentucky, Colorado, Louisiana, and Arkansas — all closely watched and hotly contested races — will all go Republican. A couple of Democratic incumbents will be unseated.
The Democrats are also going to lose the Massachusetts governor's race to the Republican Charlie Baker, a big psychological blow. I think the only bright spot for Democrats on election night will be that Scott Brown will be defeated in his bid to return to the senate, this time from New Hampshire.
Republicans will also pick up seats in the House. ElectionProjection.com currently has it at 240 Republicans to 195 Democrats. That sounds about right.
It's going to be a bad night for Democrats.