A look below at the Fix’s latest rankings of the 10 most competitive Senate races in 2016 immediately tells you one thing: Republicans have significant vulnerability. Of the 10 states considered the most ripe for a party takeover, eight are held by Republicans — including seven that President Obama won in 2008 and six that he won in 2012.

But that initial reading of the numbers is slightly deceiving. Yes, Republicans such as Sen. Mark Kirk (Ill.) and Sen. Ron Johnson (Wis.) are in a tenuous position to win second terms. But beyond that, there are no races that today you would call 50-50. Sure, Pennsylvania is a Democratic state, but Sen. Patrick J. Toomey is an able candidate, and Democrats are down on the prospect of a rerun from former congressman Joe Sestak. Sens. Kelly Ayotte (N.H.), Roy Blunt (Mo.), Rob Portman (Ohio), Marco Rubio (Fla.) and Richard Burr (N.C.) are all relatively young and have faced real races in the not-too-distant past. Yes, they sit in swing (or Democratic-leaning) states, but none of them are underdogs (or even close) yet.

And with Democrats needing a four-seat (if they win the White House) or five-seat (if they don’t) gain, simply counting on beating lots of these GOP incumbents isn’t a foolproof strategy. To give themselves a better chance at reseizing the majority, Democrats need to find ways to expand the playing field (as Republicans did last year).

The key, then, for Republicans’ hopes of staying in the majority is limiting (or eliminating) retirements in states that are tough to hold. Florida is the most obviously problematic in that regard, as Rubio has said that he won’t run for Senate reelection if he joins the presidential race, and all signs point to him running for the White House. In Arizona, Sen. John McCain insists he will be running for a sixth term, but he will be 80 on Election Day 2016, and his age ensures that retirement rumors will continue to swirl. Ditto Iowa’s Charles E. Grassley, who is running for a seventh. (Grassley will be 83 on Election Day 2016.)

If two or all of that trio wind up calling it quits for 2016 — and throw in the possibility of Sen. Rand Paul (Ky.) being forced to vacate his seat as he pursues the presidency — then that year’s playing field becomes far more fraught for Republicans. If, on the other hand, only one of those four — probably Florida or Kentucky — becomes an open seat, Republicans should feel relatively confident about their chances of holding the chamber.

The races are ranked below. The race ranked No. 1 is the one most likely to lead to a change in parties in November 2016.

10. Missouri (Republican-controlled). Democrats got their candidate this week in the form of Secretary of State Jason Kander. Kander, an Army veteran who served in Afghanistan, is 33 and may well be setting himself up for another race in the future. That's because it remains to be seen whether Blunt is actually vulnerable in a state that has been moving toward Republicans at the presidential level. At the end of last year, Blunt had more than $2 million in the bank for the race.

9. North Carolina (R). Burr has gotten on his fundraising horse in recent weeks, raising a reported $1 million at a single fundraiser last month after heading into the 2016 cycle with just $700,000 cash on hand. The big name on the Democratic side is former senator Kay Hagan, who lost by two points in November but just took a fellowship at Harvard University.

8. Florida (R). Two big Democratic names have stepped forward: Reps. Patrick Murphy and Debbie Wasserman Schultz. Murphy, at 31, is the most intriguing option here and is probably the preference of Democratic leaders who have tired of Wasserman Schultz. But Wasserman Schultz, the Democratic National Committee chairman, would bring fundraising heft to the race. The other big question, of course, is whether Rubio runs for president. If he does, he has said he won't run for a second Senate term; an open seat would be much better for Democrats.

7. Ohio (R). The contrast in Democratic candidate options here is striking. On one hand, there is Cincinnati City Council member P.G. Sittenfeld, a 30-year-old up-and-comer who is in the race. On the other is Ted Strickland, the 73-year-old former governor and House member who is, depending on whom you believe, either running or still thinking about it. Conventional wisdom suggests that Strickland would be the stronger candidate, although his long record in Ohio politics might be more of a burden than Democrats believe.

6. New Hampshire (R). Republicans have taken to labeling Gov. Maggie Hassan (D) as "U.S. Senate candidate Maggie Hassan." Democrats should be so lucky. An NBC News-Marist College poll this week showed that 70 percent of registered voters approve of her job performance. Despite that glowing review, though, she led Ayotte within the margin of error, 48 percent to 44 percent, in a potential matchup.

5. Colorado (Democratic-controlled). What we know: Sen. Michael F. Bennet (D) is vulnerable in this swing state. What we don't: what the Republican field might look like. The current thinking is that either Rep. Mike Coffman or his wife, newly elected state Attorney General Cynthia Coffman, runs — and whichever one does make a go of it is the favorite to be the nominee.

4. Pennsylvania (R). A Quinnipiac poll released this month showed Toomey leading Sestak in a rematch of their 2010 campaign. Democrats, though, hope Sestak isn't their nominee because of his unpredictability and the insularity of his campaign team. (His family members are his closest advisers.)

3. Nevada (D). Gov. Brian Sandoval (R) won't challenge Senate Minority Leader Harry M. Reid (D) — or even run if Reid retires. Republicans haven't given up trying to land the big fish, but National Republican Senatorial Committee Vice Chairman Dean Heller (Reid's Nevada colleague) offers three alternatives: state Senate Majority Leader Michael Roberson, former lieutenant governor Brian Krolicki and former State Assembly minority leader (and former Sandoval chief of staff) Heidi Gansert.

2. Wisconsin (R). Former senator Russ Feingold (D) is leaving his job at the State Department, and the timing seems to suggest that he'll seek a rematch with Johnson, who beat the Democrat by five points in 2010. Feingold has heft as a former senator, but it's worth noting that he won his previous Senate bids with 51 percent, 53 percent and 55 percent of the vote.

1. Illinois (R). Democrats are in wait-and-see mode with Rep. Tammy Duckworth, who is their preferred challenger against freshman Kirk. Duckworth has said that she is taking a "serious look" at the race. Kirk has been decidedly chesty about his reelection prospects. But a presidential year in a state as Democratic as Illinois represents Kirk's toughest race yet — by a lot. (Previous ranking: 3.)

Aaron Blake contributed to this column.