The Washington PostDemocracy Dies in Darkness

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

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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.

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 Grassley and 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)

8. Kentucky (R): Senate Minority Leader Mitch McConnell looks to be in strong position in his May 20 primary fight against businessman Matt Bevin. The general election race against Secretary of State Alison Lundergan Grimes looks considerably tougher for McConnell whose image in the state has taken a beating over the last few years. Still, this is a very Republican state at the federal level -- particularly in an environment nationally that looks to favor the GOP. (Previous ranking: 7)

7. Alaska (D): The latest TV ad from Sen. Mark Begich (D) is one of the best we have seen this cycle. It's a semi-biographical spot from veteran ad maker Mark Putnam that reminds voters of Begich's Alaska roots and his father's legacy. It's also a subtle reminder of the he's-not-of-Alaska attack pattern Democrats intend to double down on against former attorney general Dan Sullivan (R), who grew up in Ohio. The good news for Republicans is that Sullivan has emerged as a clear frontrunner in the GOP primary, raising heaps of cash and lining up both establishment and tea party support (the Club For Growth recently endorsed him). Lt. Gov. Mead Treadwell (R) is shaking up his campaign staff, almost never a sign things are going well. Americans For Prosperity has staked out a big presence in Alaska, but Democrats have vowed not to be outspent, given how relatively cheap it is to advertise in the sparsely populate state. (Previous ranking: 5)

6. North Carolina (D): The Koch brothers-backed group Americans for Prosperity has been barraging Sen. Kay Hagan (D-N.C.) early in 2014, and polls suggest it’s paying dividends. One poll showed Hagan with just a 33 percent approval rating and 49 percent disapproval. Other don’t show it quite so ominous, but still not great for Hagan. State House Speaker Thom Tillis (R) meanwhile, hasn’t proven an ideal standard-bearer, with a report Thursday showing his online bios over-stated his college degree (Previous ranking: 8)

5. Arkansas (D): Democrats are remarkably optimistic about Sen. Mark Pryor's chances given the Natural State's movement toward Republicans and the fact that Rep. Tom Cotton appears to be one of Republicans' strongest recruits in the country. Pryor is a proven candidate with a well known name. But, we remain skeptical that will be enough to insulate him from what Cotton -- and groups like Americans for Prosperity -- have and will continue to hammer him with. (Previous ranking: 4)

4. Louisiana (D): Sen. Mary Landrieu (D) has been getting barraged by AFP and she's received relatively light air cover from Democratic allies. The incumbent is taking matters into her own hands, making a heft air reservation for the Spring/Summer -- an apparent recognition of the heavy toll the attack ads are exacting on her image. Polls, meanwhile, show her locked in a close race against Rep. Bill Cassidy (R). In short, this looks like the race Democrats are worried about most. Landrieu, for now, replaces Pryor as the most vulnerable senator on the map. (Previous ranking: 6)

3. Montana (D): Appointed Sen. John Walsh (D) probably helps Democrats with his newfound incumbency (he was running before former senator Max Baucus was confirmed as ambassador to China). This is still a tough hold in a red state, but Democrats have had success here in recent years, with a governor and two senators from their party. And don’t forget that President Obama came within three points of pulling an upset in 2008. Rep. Steve Daines is the GOP frontrunner. (Previous ranking: 3)

2. West Virginia (D): This open seat race has been pretty quiet this cycle. That's probably good news for Republicans, who continue to have the upper hand in this state where the Obama administration is deeply unpopular. The most recent polling showed Rep. Shelley Moore Capito (R) with a double-digit advantage over Secretary of State Natalie Tennant (D). (Previous ranking: 2)

1. South Dakota (D): The political career of former senator Larry Pressler (R) is something to behold. He briefly ran for president at the tender age of 37 and even flirted with a run for mayor of Washington, D.C. At 71, the former three-term senator is running this year as an independent, and he’s basically the one thing that stands between his former party and an easy pickup. National Democrats don’t feel great about their likely nominee, former congressional aide Rick Weiland, but if Pressler can run a real campaign and steal votes from former governor Mike Rounds, who knows? (Previous ranking: 1)

Aaron Blake and Sean Sullivan contributed to this report.