Sen. Mark Pryor (D-Ark.) is a proven candidate with a well-known name. But we remain skeptical that it will be enough to insulate him from what Tom Cotton — and groups like Americans for Prosperity — have and will continue to hammer him with. (Danny Johnston/AP)

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. However, GOPers 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 number 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 such as Arizona (John McCain) and Iowa (Charles E. 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 F. Bennet in Colorado and Harry M. 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 Democrats 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 11 Senate races most likely to switch parties this November. The No. 1 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.

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 the offense.

8. Kentucky (R): Senate Minority Leader Mitch McConnell looks to be in a 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 past few years. Still, this is a very Republican state at the federal level — particularly in an environment nationally that looks to favor the GOP.

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 Alaskan 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 front-runner in the GOP primary, raising heaps of cash and lining up support from both the establishment and the tea party.

6. North Carolina (D): Americans for Prosperity, backed by the Koch brothers, has been barraging Sen. Kay Hagan (D), 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. Meanwhile, state House Speaker Thom Tillis (R) hasn’t proven an ideal standard-bearer for his side.

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 it will be enough to insulate him from what Cotton — and groups like AFP — have and will continue to hammer him with.

4. Louisiana (D): Sen. Mary Landrieu (D) has been getting barraged by AFP, and she’s received relatively little air cover from Democratic allies. The incumbent is taking matters into her own hands, making a hefty ad buy 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).

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, and Rep. Steve Daines is a solid GOP front-runner.

2. West Virginia (D): The most recent polling showed Rep. Shelley Moore Capito (R) with a double-digit advantage over Secretary of State Natalie Tennant (D). A very tough hold for Democrats.

1. South Dakota (D): National Democrats don’t feel great about their likely nominee, former congressional aide Rick Weiland, but if former senator Larry Pressler, who is running as an independent, can steal votes from former governor Mike Rounds (R), who knows?

Aaron Blake and Sean Sullivan contributed to this report.