Former Democratic Montana Gov. Brian Schweitzer’s decision not to seek his state’s open U.S. Senate seat has boosted Republican hopes of gaining control of the upper chamber in 2014, with New York Times’ statistical guru Nate Silver now giving near even money to the possibility. But the more important question isn’t whether Republicans can eke out control of the Senate in 2014, but whether they could maintain control 2016, when they face a much more intimidating map.

In American politics, the flip side of a party having an election cycle in which their candidates win a lot of Senate seats is that six years later, they’ll be forced to defend those seats when political conditions might be less favorable. In 2014, Democrats will have to defend a batch of Senate seats that were won during President Obama’s landslide 2008 victory. That gives Republicans an opening.

The problem for Republicans is that, even if they win a narrow 51-seat Senate majority in 2014, two years later, they’ll have to defend the seats they picked up or held onto during their own 2010 wave election. That means potentially difficult contests in Arizona, Florida, Illinois, Missouri, New Hampshire, Ohio, Pennsylvania and Wisconsin.

The reason why 2016 is more important is that the Senate elected that year will be the one in office when a new president is sworn in in 2017. Even if Republicans win control of the Senate in 2014, their ability to accomplish anything will be limited as long as Obama is in office. Their first chance to advance an agenda would be if a Republican is elected president in 2016. But if control of the Senate swings back to Democrats that year, then the new GOP president is going to have a tough time getting his or her agenda passed and nominees confirmed.

Obviously, it’s difficult to know what the political environment will be like that far into the future. But Republicans giddy about the party’s Senate prospects in 2014 shouldn’t lose sight of the fact that 2016 will likely be a much more daunting challenge.