Sen. Claire McCaskill (D-Mo.) is more vulnerable than ever in the wake of a scandal over unpaid taxes on her private plane. (Alex Wong/Getty Images)

National Democrats have spent the first few months of the election cycle trying to wrap their arms around the depth of the challenge they face, urging wavering lawmakers to make their decisions early (five have announced they won’t seek reelection) and working with those who are running on strengthening their position.

While it’s long been expected that McCaskill will have a serious race in 2012 — the state went narrowly for John McCain in 2008 and McCaskill won in 2006 with 50 percent — she has also been regarded as one of Democrats’ most able politicians, someone who will do everything she needs to do to win.


The plane episode suggests that McCaskill’s political radar — ahem — might not be as finely tuned as many strategists had long believed. Democrats argue that McCaskill’s forthright self-reporting of the tax issue has and will continue to win her kudos with voters. It might . But it also has handed Republicans a made-to-order attack that fits quite nicely in the space of a 30-second television commercial.

Simply put: McCaskill is significantly more vulnerable today than she was a week ago. (The Cook Political Report formalized that reality on Thursday when it moved the race from “lean Democratic” to “toss up”.)

Senate Democrats can’t have more unforced errors like the one committed by McCaskill if they want to hold on to the Senate majority. They are trying to thread a very fine political needle to hold the majority in 2012 and need the breaks to go their way.

Below are the ten seats considered most likely to switch parties in 2012. The number one race is the likeliest to flip party control. Agree or disagree with our picks? The comments section awaits.

To the Line!

Coming off the Line: Arizona

10. Nevada (Republican controlled): Sen. John Ensign’s (R) retirement announcement and Rep. Dean Heller’s (R) subsequent entry into the race considerably improves the party’s chances of holding the seat in 2012. But Nevada is a swing state, and Democrats expect to be very competitive. The names in the mix are Rep. Shelley Berkley, Secretary of State Ross Miller , state Attorney General Catherine Cortez Masto and wealthy businessman Byron Georgiou. Keep an eye on Heller’s first fundraising report, due out in mid-April; it’s just half a month worth of fundraising, but if he puts up a big number, that may give some Democratic candidates pause. (Previous ranking: 8)

9. New Mexico (Democratic-controlled): Republicans got, arguably, their best general election candidate in the race in the form of former Rep. Heather Wilson. But Wilson, who lost a 2008 Senate primary to the more conservative Rep. Steve Pearce, will again have to prove her conservatism to the base. Hoping to fill the vacuum are Lt. Gov. John Sanchez, who is close to getting in the race, and former congressional candidate Greg Sowards. Sanchez, in particular, has a real opportunity but plenty to prove. Meanwhile, Democrats are still waiting on Rep. Martin Heinrich and state Auditor Hector Balderas to make up their minds. Pearce and Rep. Ben Ray Lujan (D) have also been mentioned. (Previous ranking: 10)

8. Ohio (D): The Republican field to face Sen. Sherrod Brown (D) is slowly starting to form. State Treasurer Josh Mandel appears to be leaning toward the race and former Secretary of State Ken Blackwell is taking a look at running too. A new Quinnipiac University poll shows Brown in strong shape; 45 percent of voters think he deserves a second term and he leads a generic Republican challenger by 16 points. But, Ohio went strongly for Republicans in 2010 and both sides expect the race to tighten up. (Previous ranking: 9)

7. Florida (D): Rep. Connie Mack’s (R) announcement Friday that he would not run for the state’s Senate seat means the GOP primary will be slightly less crowded. State Senate President Mike Haridopolos is in the race and raising money, but former Sen. George LeMieux and former state Rep. Adam Hasner appear undeterred and likely to run. The winner of the primary gets Democratic Sen. Bill Nelson , whose approval rating in a recent Quinnipiac poll was 45 percent. That isn’t great, but keep in mind: his disapproval was at just 21 percent. That means even Republicans don’t necessarily dislike him. We’ll see how vulnerable he actually is as the cycle progresses. (Previous ranking: 6)

6. Massachusetts (R): The odd shuffle done by Sen. Scott Brown (R) in the past week over Planned Parenthood exemplifies the difficulty of being a Republican in a Democratic state. Yet his maneuvering seems to be working. A top-tier Democratic candidate has yet to emerge in this race (although Democrats promise one is on the way) and polls suggest Brown starts from a position of strength. A recent Western New England College Polling Institute survey found him with a 53 percent favorability rating and double-digit leads over two potential rivals. (Previous ranking: 7)

5. Montana (D): Sen. Jon Tester is in a statistical dead heat with Republican Rep. Denny Rehberg, according to a new Mason-Dixon poll. That same poll showed Tester with a 12-point lead among independents and, given the Last Best Place’s Republican leanings, he’ll have to maintain that edge (or close to it) to win. (Previous ranking: 5)

4. Virginia (D): The near-certain candidacy of Democratic National Committee Chairman — and former Virginia governor — Tim Kaine is a major recruiting victory for the Democratic Senatorial Campaign Committee. Kaine is, without question, the party’s strongest candidate but victory is far from certain. Former senator George Allen is the likely Republican nominee and he has shown an ability to win statewide in the Commonwealth before. This will be a — if not the -- marquee race of the 2012 Senate cycle. (Previous ranking: 3)

3. Missouri (D): Sen. Claire McCaskill was vulnerable before this week; now she’s on shakier ground than ever. The revelation that she failed to pay almost $300,000 in taxes – on a private plane that she used for taxpayer-funded trips, no less – is a big blow to her reputation as an ethics watchdog. The only good news for McCaskill is that so far, the GOP field remains something short of star-studded. But this seat is a top target for Republicans, who will surely do everything they can to keep the plane story in the news for the next 20 months. (Previous ranking: 4)

2. North Dakota (D): This open seat contest is one of the quietest in the nation as both parties continue to try to sort out their fields. Freshman Rep. Rick Berg (R), despite just arriving in Washington, is seen as a potentially strong candidate while Democrats are hoping to convince the man Berg beat in 2010 — Earl Pomeroy — to run. Regardless of the candidates, Republicans will almost certainly be favored to take over the seat Sen. Kent Conrad (D) is vacating due to the conservative tilt of the state. (Previous ranking: 2)

1. Nebraska (D): Sen. Ben Nelson still looks like the most vulnerable man (or woman) in the Senate, but Republicans are likely headed for a bruising primary, which could give the Democratic incumbent some hope. State Treasurer Don Stenberg (R) announced his candidacy earlier this month and instantly started alluding to some liberal things that state Attorney General Jon Bruning (R) wrote in college. Nelson’s got problems, no doubt, but if Republicans bludgeon each other in the primary and the nominee emerges wounded, don’t count out the battle-tested senator. At the same time, Nebraska’s primary is in May, and six months is a lot of time to recover for whomever is the Republican nominee. (Previous ranking: 1)

With Aaron Blake and Rachel Weiner