If the Republicans overplay their hand as expected and bog down the Senate wasting time with Obama investigations, trying to repeal the ACA, and putting poison pills in bills, it will set the Democrats up for a nice rebound in 2016 when there are 10 Democrat Senators up for reelection in contested states compared to 24 Republicans.
Reagan lost the Senate to the Democrats in his 2nd midterm by a 55-45 margin. so these swings are cyclical. It helps clear out some of the dead wood and forces the losing party to reevaluate their priorities and message.