Sure, Republicans can win the Senate in 2014. But can they keep it in 2016?

Momentum is with Republicans in the fight for the Senate majority this fall. A combination of late-breaking recruitment successes, a national environment tilting toward the GOP and the raw number of seats for each side at stake have combined to tip the balance in favor of Republicans gaining the six seats they need to retake control.

The United States Capitol. (Photo by Win McNamee/Getty Images)

So, let's say Republicans retake the Senate this fall. Can they keep it in 2016?

Much of that depends on just how many seats they win in November. Yes, they technically need six seats for the majority. But, they probably need to pick up in the neighborhood of eight or even nine seats in order to ensure themselves a fair shot at holding the Senate for more than two years.

Here's why: There are 23 Republican seats up compared to just 10 for Democrats in 2016. (This is the class that got elected in 2010, a great year to be a Republican.) And it's not just the raw numbers. Much like how the geography of the Senate map in 2014 favors Republicans, there are a numbers of GOP-held seats in traditionally Democratic states in 2016. Republicans in Illinois, New Hampshire, Ohio, Pennsylvania and Wisconsin all would have to run for reelection in a presidential year in states that President Obama won in 2008 and 2012. Then there are potential retirements in places like Arizona (John McCain) and Iowa (Chuck Grassley). (Before we get angry calls, yes, we know that Grassleyand McCain have hinted at running again.But, 2016 is a long way off. And McCain will be 80, Grassley 83.)

By contrast, there are only two Democratic seats -- Michael Bennet in Colorado and Harry Reid in Nevada -- that appear to be in any initial danger. And, even if some longer-serving Democratic incumbents decide to call it quits, they tend to represent states like California and Maryland where their party would be heavily favored to win an open seat.

Add it all up and you begin to see why, unless Republicans can get to 53-ish seats, they might find their Senate majority very short-lived.

Below we rank the eleven Senate races most likely to switch parties this November. The number one race is considered the most likely to switch parties.

10. Colorado (Democratic-controlled): Rep. Cory Gardner's (R) decision to reverse course and run against Sen. Mark Udall (D) turns this race from what was rapidly looking like a missed opportunity for Republicans into a real race. Gardner's entry led a number of primary challengers to drop out, strengthening his hand against Udall. Polling suggests Udall is vulnerable -- as is any Democrat in this environment in a swing state -- but it remains to be seen whether Gardner can make good on his potential. (Previous ranking: Unranked)

9. Michigan (D)/Georgia (Republican-controlled): Michigan Republican Terri Lynn Land and Georgia Democrat Michelle Nunn are running about even with the opposing party contender(s), welcome news for strategists in both parties looking to maximize the turf on which they can go on offense. A automated poll from SurveyUSA showed businessman David Perdue leading the Georgia Republican pack with 29 percent support. Perdue's success could be due to his heavy spending and voter familiarity with the Perdue name. (His cousin Sonny Perdue used to be governor.) It looks like the GOP primary is headed to a runoff, which would be good news for Nunn. (Previous ranking: 10)