I like Charlie Cook’s stuff, but there’s a glaring hole in the logic here:
…Democrats have fared well in Senate races when the presidency was up for grabs. In 2008 and 2012, they picked up eight and two seats, respectively. Their gain in 2012 wasn’t larger because they’d already picked up four seats in 2000 and six more in 2006—the two previous times this class of senators had faced voters—leaving fewer additional seats within their reach.
Conversely, Republicans did wonderfully in the midterm elections of 2010 and 2014, when they picked up six and nine seats, respectively. Add in the impact of the political toxicity surrounding Obama in 2010, and Republicans had a hurricane-force wind at their backs. The class of senators who are up for reelection in 2016 were the beneficiaries, but now they must face an electorate that is demographically more daunting.