The magic number for Republicans in 2012 is four. The GOP has to pick up just four Senate seats order to gain control of the upper chamber, making this the fourth straight election where the balance of congressional power lies in just a handful of races. And while the election is still more than a year away, the battle lines are already starting to be drawn.
A total of 33 seats will be up for grabs in the 2012 election, 23 of which are held by Democrats. Republicans will have open opportunities in six of those 23 states where the Democratic incumbent is retiring.
Montana, Missouri, Nebraska and North Dakota are shaping up to be some of the closest Democrat-defended races, with Nevada and Massachusetts presenting possibilities for a switch from red to blue.
Only two Republicans are retiring, Kay Bailey Hutchison of Texas and Jon Kyl of Arizona, although the GOP will likely hold on to those traditionally red-state seats. With Tea Party popularity high in many states, GOP senators could face tough primary races.
The still-floundering economy could mean trouble for Democrats who have twice as many senators up for re-election than Republicans. But the extra attention and resources of the Obama campaign is expected to turn out much larger numbers of Democratic voters than went to the polls in 2010, when Republicans gained six seats.
Here's a look at the Senate races that could determine which party will have control after November 2012.
Incumbent Claire McCaskill, a first-term Democrat, faces one of the toughest re-election bids in the country. In 2006, she narrowly defeated then-incumbent Jim Talent with 49.6 percent of the votes. And in a red-leaning state that voted for Jon McCain in 2008, McCaskill is polling just slightly ahead of her two leading Republican opponents, Rep. Todd Akin and former state treasurer Sarah Steelman.
As of June 30, 2011, McCaskill had amassed $2.8 million for her re-election campaign, more than twice as much as her closest competitor, Akin, who had $1.2 million cash on hand.
Earlier this year, McCaskill faced criticism after being accused of spending taxpayer money to fly her private plane around the state. She was also accused of failing to pay $300,000 in property taxes for the plane.
After the controversy broke, McCaskill quickly paid the back taxes, reimbursed the state treasury for the flights and decided to sell her plane.
"I have convinced my husband to sell the damn plane," McCaskill said in a March conference call with reporters. "I will never set foot on the plane again."
The Montana Senate race could be both one of the closest in the country and one of the most expensive. Democratic incumbent Jon Tester stockpiled almost $4 million before June 30 and his Republican opponent, Rep. Denny Rehberg, has close to $2 million waiting in his war chest for a race that is expected to cost more than $20 million.
Tester, who served in the Montana state Senate for almost a decade before heading Capitol Hill, won his U.S. Senate seat by a razor-thin margin in 2006, beating incumbent Conrad Burns by less than one percent. Rehberg, a member of the Tea Party, has held Montana's only U.S. congressional seat since 2001.
Despite the election being more than a year away, the battle between Tester and Rehberg has already commenced with ads blasting Tester for being in lock-step with President Obama, who is not particularly popular in Montana, and commercials attacking Rehberg for not being environmentally friendly.
Democratic incumbent Ben Nelson faces an uphill battle in 2012 to keep his Senate seat, which many political pundits have pinned as one of the Democrats' most vulnerable. The two-term senator has become a Republican target after casting the 60th vote Democrats needed to pass President Obama's health care law. But with three Republican challengers, the Nebraska race is heating up early.
State Attorney General Jon Bruning, who is considered the race's front-runner, has snagged endorsements from former Arkansas Gov. Mike Huckabee and the Tea Party Express. Bruning out-fundraised his three primary challengers, state Treasurer Don Stenberg, state Sen. Deb Fischer and businessman Pat Flynn, although Fischer's late entrance into the race in June exempted her from the June 30 campaign filing deadline. Bruning's cash on hand still lags behind Nelson's by about $1.7 million.
This is Stenberg's fourth Senate bid. He lost his three previous attempts and most notably fell to Nelson in the 2000 Senate race.
Sen. Kent Conrad's decision to retire after four terms has left Democrats on unstable ground in North Dakota, a state that has voted for Republican senators in 21 of 37 elections since 1914, according to a University of Minnesota "Smart Politics" analysis.
Freshman Rep. Rick Berg, a Tea Party favorite, will battle retired Navy commander Duane Sand, also a Tea Party supporter, in the state's GOP primary. This will be Sand's fourth congressional run after two unsuccessful House runs and one loss to Conrad in 2000.
The fight for retiring Virginia Sen. James Webb's seat is shaping up to be a battle royale between two former governors, Republican George Allen and Democrat Tim Kaine. The most recent poll done by Quinnipiac University in June showed the two candidates neck-and-neck, with Kaine leading Allen by just one percentage point.
Allen held the Senate seat from 2001 to 2007, until he was defeated by Webb. Allen was also a member of the U.S. House in the early 1990s before successfully running for the governorship.
House Majority Leader Eric Cantor, R-Va., endorsed Allen earlier this month as did Rep. Morgan Griffith, R-Va., and Rep. Bob Goodlatte, R-Va. As of June 30, Kaine, former Democratic National Committee chairman, had slightly more cash in his war chest, reporting about $1.8 million cash on hand compared to Allen's $1.6 million.
With four-term Sen. Herb Kohl retiring, the Wisconsin's Senate race is now a prime target for Republicans and presents a real opportunity for the GOP to pick up one of the four seats it needs to gain control of the Senate. While no candidates have formally announced, former Wisconsin Gov. Tommy Thompson will likely be the Republican nominee and Rep. Tammy Baldwin is expected to announce her bid for the Democrat ticket. Baldwin was the first Wisconsin woman elected to Congress and, if she wins the Senate seat, she would be the first openly-gay senator ever elected.
Speculation that former Sen. Russ Feingold was going to run was squashed last week when Feingold announced he planned to stay in the private sector. Feingold's decision not to run paves the way for Baldwin to take the Democratic nomination.
Democratic Rep. Shelley Berkley will face off against Republican incumbent Sen. Dean Heller in a 2012 Senate race that is expected to be one of the toughest in the nation. Heller, a former U.S. House member, was appointed to serve the remainder of former Sen. Jon Ensign's term earlier this year.
While Heller's House district, which he served for almost two terms, lies in the more rural northern part of Nevada, Berkley's district is centered in Southern Nevada and includes Las Vegas. Internal polling from Heller's campaign showed he was beating Berkley by double digits, but a poll from Berkley's campaign last month showed the congresswoman was up by about four points.
Steve Ansolabehere, a government professor at Harvard University, said the economy would probably be the biggest factor in this Nevada Senate race because the state is still reeling from high unemployment and high foreclosure rates. Ansolabehere predicted that if the economy picks up, Berkley might prevail, but if it continues to flounder, Heller will likely have the upper hand.
In a state that has voted Democratic in the past six presidential elections, Republican incumbent Scott Brown could have a tough time defending the late Ted Kennedy's Senate seat in 2012. Brown came from behind in the polls to win a 2010 special election to succeed Kennedy, becoming the first Republican to win a U.S. Senate seat in Massachusetts since 1972. His victory stripped Democrats of their filibuster-proof majority in the upper chamber.
Three Democrats have already announced their bids to unseat Brown, including Alan Khazei, co-founder of the non-profit group City Year, activist Bob Massie and Newton, Mass., Mayor Setti Warren. After forming an exploratory committee last week, former White House official and Harvard Professor Elizabeth Warren is also expected to throw her name in the ring. Warren's move was praised by liberal Democrats who commend her record of taking on Wall Street by creating the Consumer Financial Protection Bureau, which creates and enforces financial regulations.