As you can see, if the GOP wins a bare majority in 2014, the odds are very, very good that the Senate will revert back to Democratic hands in 2016. In fact, if GOP gains are confined to the “traditional seven” Democratic races (the three open seats and the four incumbents in states Mitt Romney carried), they’re still favored to lose the chamber two years later. On the other hand, if Republicans get to 54 seats, their chances of retaining control are very good, and given the horrific playing field for Democrats in 2018, they would be extremely unlikely to lose it that year.
Perhaps of more interest, if Republicans gain only a seat or two, a filibuster-proof Democratic majority in 2016 is at least plausible. If Republicans break even or lose seats — and remember, no one thought that Republican losses were plausible at this point in 2012 — a filibuster-proof Democratic majority might even be likely in 2016. A year good enough to net Democrats six or more Senate seats would probably given them control of the House as well, giving them an unlikely trifecta for the second time in eight years.