Charlie Cook thinks it’s in the Senate – and could be permanent:
The reason next year is so make-or-break for Senate Republicans is because in 2016, when all of the seats they won in 2010 come up—they netted a six-seat net gain that year—there will be 24 GOP seats up, compared with only 10 for Democrats, leading to some serious Republican overexposure. Seven of the 24 GOP senators up are hailing from states that Obama carried in 2012. After having had plentiful Democratic targets in 2012 and 2014, it will be Republicans in 2016 who will have the most incumbents in the crosshairs.
Kilgore chimes in:
2016, moreover, being a presidential election year, is likely to produce the kind of relatively high turnout that tends to help Democrats disproportionately. So for those Republicans who did not consider 2012 a “now and never” opportunity after which conservatives would be submerged in a wave of dusky looters, 2014 is a very big deal. When one seeks a radical counter-revolution overturning decades of “socialist” policies, control of the entire federal government is a must. The Senate could be gone for a good while if GOPers don’t win it back next year.
If Republicans do gain control of the Senate after the midterms, it’s also quite possible that their majority will be so small that it could quickly be wiped out when some of the class of 2010 has to run during a presidential election year. Republicans are running into a recurring problem where they have to run up huge wins in the midterms just to be able to absorb their failures in the presidential years, so that even an average midterm performance becomes inadequate.