Republicans are playing lots of defense this cycle, with four of the top five seats likeliest to flip belonging to the GOP. Overall, the GOP has to defend 24 seats to just 14 for Democrats.

Pennsylvania Gov. Tom Corbett (R) remains the most vulnerable incumbent in the country. Maine Gov. Paul LePage (R) moves up to the No. 2 spot, meanwhile, as Republicans' best pickup opportunity is still in Arkansas.…

And now, to our list of the 15 seats most likely to change party control. As always, No. 1 is most likely to flip.

15. Minnesota (Democratic-controlled): There is no shortage of Republicans seeking to challenge Gov. Mark Dayton (D), with four having signed up for the GOP primary. The question is whether Dayton is actually vulnerable in a blue-leaning state. Dayton, who struggled in his one term as senator, has fared better in the governor’s mansion, but recent polls peg his approval rating around 50 percent. Minnesota has elected plenty of Republican governors, so don’t think it won’t be a competitive race. (Previous ranking: 15)

14. Arizona (Republican-controlled): Gov. Jan Brewer (R) said recently that she will hold off on making a decision about whether she will challenge the state constitution to run for another term until next year. If she doesn't try to make another bid, GOP focus will center on Secretary of State Ken Bennett, state Sen. Al Melvin, state Treasurer Doug Ducey and Mesa Mayor Scott Smith. (Previous ranking: 14)

13. Massachusetts (D): Might we see a rematch of the 2010 Senate special election? Attorney General Martha Coakley (D) may run, and former senator Scott Brown (R) says to "stay tuned." Coakley is popular despite her disastrous Senate bid, so while Republicans have had some Bay State success in governor's races, if she runs, a GOP pickup will face tough odds. (Previous ranking: 12)

12. Wisconsin (R): Gov. Scott Walker (R) saw his approval rating dip below 50 percent recently. If Democrat Mary Burke jumps in the race against him, Walker will be up against a candidate with a business background and a personal fortune to spend. We're keeping a close eye on this contest, but for now, Walker still has the upper hand. (Previous ranking: 13)

11. Colorado (D): Gov. John Hickenlooper (D) shrugged off the notion that he might be vulnerable on the sidelines of the recent National Governors Association meeting. But it's hard to ignore two polls from recent months that show him in tough races against Republicans. The GOP field doesn't look impressive, and on the whole, Hickenlooper is still the favorite to be reelected. But his numbers are worth watching in the coming months. (Previous ranking: 9)

10. Connecticut (D): Former ambassador to Ireland Tom Foley (R) just about upset Gov. Dan Malloy (D) in 2010, but his close call hasn't earned him a free ride in the GOP primary this year. State Senate Minority Leader John McKinney is challenging Foley for the GOP nod, and Danbury Mayor Mark Boughton recently joined them. Malloy, meanwhile, has middling approval ratings. (Previous ranking: 8)

9. Ohio (R): The remarkable turnaround of Gov. John Kasich (R) will be one of the biggest stories of the cycle if the once woefully unpopular governor hangs on to his job. Democrats, meanwhile, feel good about Cuyahoga County Executive Ed FitzGerald (D); the question for him is whether he can keep pace in the money chase. (Previous ranking: 10)

8. Illinois (D): Attorney General Lisa Madigan’s (D) decision not to run was a huge break for Republicans. She would have been a heavy favorite in the Democratic primary against Gov. Pat Quinn and, if she won the nomination, would be a shoo-in in the general election. Alas, she passed on another big-time race, leaving Quinn to battle former White House chief of staff Bill Daley for the Democratic nomination. Republicans hope Quinn somehow pulls it out, given his approval rating, at last check, was somewhere in the 20s. (Previous ranking: 11)

7. South Carolina (R): Gov. Nikki Haley (R) is set to kick off her reelection campaign – something that was somewhat uncertain. Democrats think they can defeat her, but as we pointed out this week, this remains a tough race for them to win in a red state. The good news for Democrats is they’ve got state Sen. Vincent Sheheen, the guy who almost did it in 2010. But we have to believe getting over the hump won’t be easy. (Previous ranking: 7)

6. Virginia (R): Former Democratic National Committee chairman Terry McAuliffe (D) has been forced to confront tough stories about his former car company in recent weeks. Attorney General Ken Cuccinelli (R), meanwhile, has had to answer questions about gifts he received from the donor at the center of a scandal involving Gov. Bob McDonnell (R). Look for a negative, nasty campaign down the stretch. The side that does a better job hitting the opposition may well be the victor. (Previous ranking: 6)

5. Michigan (R): The question here is whether Gov. Rick Snyder (R) can tread the same path Walker and Kasich did, bracing for impact after signing divisive legislation (in Snyder's case, a "right to work" bill) and relying on time to heal the damage inflicted among moderates. On the other side of the contest, Democrats are enthusiastic about former congressman Mark Schauer (D). (Previous ranking: 5)

4. Arkansas (D): It's been an awfully good year for former congressman Mike Ross (D). He's been raising money at a very fast pace, and recently scared away primary opponent Bill Halter. Arkansas is still the GOP's best pickup opportunity, but Ross is doing everything and more to make it tough for them. (Previous ranking: 4)

3. Florida (R): Before you write off Gov. Rick Scott (R), remember that he will have a ton of money to spend, survived a tough primary and general election in 2010, and has gained some ground, according to a poll from earlier this summer. To be clear, Scott remains one of the most vulnerable incumbents in the country, and Charlie Crist would be a formidable Democratic foe. But for now, Scott is in a better position right now than the two governors below on this list. (Previous ranking: 2)

2. Maine (R): Rep. Mike Michaud (D) made his campaign official this week. Let the three-way race begin! Democratic polling shows Michaud up over unpopular Gov. Paul LePage (R), even with independent Eliot Cutler in the mix. Cutler will be a pain in Democrats' side this cycle, but party strategists feel better about Michaud than they did about Libby Mitchell (D) in 2010. That year, Mitchell finished third behind Cutler and LePage. (Previous ranking: 3)

1. Pennsylvania (R): It's tough being Gov. Tom Corbett (R) these days. Not only are his polling numbers lousy, Republicans in the state have been talking about finding a replacement for him, according to one report. Nothing has changed since out last round of rankings to suggest that Corbett is not the most endangered governor in the country. (Previous ranking: 1)