The problem facing Republicans is that while this year Democrats are forced to defend Senate seats that were last won during Obama’s 2008 landslide, in 2016, the reverse will be true. Republicans will be forced to defend Senate seats they won in 2010, one of the best years for the GOP in the party’s history.

The vulnerable Republican seats include Democratic-leaning states of Illinois, Pennsylvania, Wisconsin and typical swing states of New Hampshire, Ohio and Florida (where it’s unclear how Sen. Marco Rubio’s presidential ambitions might interact with the Senate race). Furthermore, Sens. John McCain and Chuck Grassley will both be octogenarians by the 2016 election, and may choose to retire, creating wide-open races in Arizona and Iowa. McCain could face a tough re-election if he does choose to run again. On the Democratic side, the only seats that look potentially vulnerable are the ones in Colorado and Nevada – if Republicans could find a viable candidate to challenge Senate Majority Leader Harry Reid.