Congressional Dem Incumbents: Who Are the Most Vulnerable?

The betting site Intrade gives Republicans a better than 50% chance of taking control of the U.S. Senate after the November elections. After picking up seven Senate seats in 2010 -- including Scott Brown’s victory in the Massachusetts special election -- the GOP needs a net four seats to assure control after 2012. If the Republicans regain the White House, a net gain of three would be sufficient as the vice president serves as tiebreaker.

Twenty-three of the 33 Senate seats being contested in 2012 are presently held by Democrats. Of that number, 12 seats currently held by Democrats are potentially at risk. Seven of these 12 are open-seat races with the Democratic senator retiring. The seven seats: North Dakota and Nebraska (where the GOP candidate is favored); Virginia and Wisconsin (which are considered tossups); and Connecticut, Hawaii, and New Mexico (which are currently rated as leaning Democratic). New Mexico looks the closest of the three.

The five Democratic incumbents at greatest risk of losing their seats, starting with the most vulnerable: Claire McCaskill (Missouri), Jon Tester (Montana), Bill Nelson (Florida), Sherrod Brown (Ohio), and Debbie Stabenow (Michigan).

It is more difficult to oust an incumbent than to win an open-seat race, whether for the Senate or the House. This is particularly true for the Senate, where incumbents have six years to build up a re-election war chest and build ties with the state’s voters. That said, the most vulnerable incumbents tend to be first-termers, and McCaskill, Tester, and Brown all fall in that category. All three won office in 2006, a very big year for Democrats around the country. McCaskill and Tester won very narrow victories: Tester by 1% (3,500 votes); McCaskill by 3% (48,000 votes). Nelson and Stabenow are both in their second term and neither faced a serious challenge in 2006.

McCaskill appears to be in the biggest trouble of any incumbent of either party. She trails all of her potential rivals on the Republican side, and the biggest gap is with Sarah Steelman, who currently leads the GOP contest for the Senate nomination. Missouri has also trended Republican in the last decade. It was the only Bush state of ten targeted by the Obama campaign that McCain won. The GOP won by huge margins in the governor and Senate races in 2010.

Jon Tester is also trailing by a small margin in Montana, where he is running against GOP House member Denny Rehberg, who has also won statewide elections (Montana has only one U.S. House seat). Montana was carried by McCain by 3% in 2008, but recent polls show Romney ahead of Obama by a bigger margin this year.

Ohio is a race in which the Democrats did not expect a fight, but that is no longer the case. Josh Mandel -- a 34-year-old Iraq war veteran and Ohio treasurer who won decisively in a statewide race in 2010 -- is a tireless campaigner. He has begun to close the gap with Brown, and is tied with him in one poll of likely voters. Brown remains the favorite.

Ohio will get a lot of attention in the presidential race. If Romney carries the state (essential to his chances of winning), that would boost Mandel’s chances, as would Romney picking Senator Rob Portman as his VP choice.

In Florida, polls are all over the place but Nelson is ahead of all possible opponents. His lead is smallest against Congressman Connie Mack, the likely winner of the GOP nomination. Florida is another tossup state for the presidential race -- a state that is slightly more conservative than Ohio -- and Mack has a near 50% chance of winning. Just as in Ohio, Mack would benefit if Romney chose Florida Senator Marco Rubio to be his running mate.