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.
— Charlie Cook, noting the 2014 midterm elections may the the GOP’s last chance to take back control of the Senate for years.