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.
All of this is to say that Republicans really have to do well in the Senate elections in 2014, largely because they will have few opportunities for gains in 2016, a year in which they will be playing defense, not offense. This means that Republicans cannot nominate some of the more exotic candidates that they nominated in Delaware and Nevada in 2010, or weak candidates with weak campaigns as they did that year in Colorado. Comparable candidates to Todd Akin or Richard Mourdock, the 2012 Missouri and Indiana candidates whose nominations effectively meant that the GOP seized defeat from the jaws of victory in multiple states, should be avoided. So, the 2014 Senate elections really are important.