Wisconsin

If recall organizers have collected as many signatures as they say they have, Republican Gov. Scott Walker will likely face a recall election in spring or summer 2012. Recalls rarely succeed, but Walker's controversial budget that curbed public employees' collective bargaining power stirred up voter opposition in 2011 and resulted in the successful recalls of two Republican state senators.

Ohio

Ohio is a critical Rust Belt swing state that has felt acute unemployment as manufacturing jobs have evaporated. When the state ousted its Democratic governor in 2010 and brought in Republican John Kasich, Ohio GOP Chairman Kevin DeWine said, "Today we kicked down Obama's firewall"-but voters in 2011 overturned a major Kasich measure curbing collective bargaining.

Michigan

Twenty-one percent of the population of Michigan is Roman Catholic, among the highest rates in the country, according to the Official Catholic Directory released in 2008. Obama won the support of a majority of Catholic voters in 2008, but his administration has a strained relationship with the Catholic Church. The state also holds its Republican primary on Feb. 28, potentially giving a candidate momentum heading into Super Tuesday on March 6.

Virginia

Obama turned this red state into a blue state in 2008, but a year later Virginians elected Republican Governor Bob McDonnell, and Republicans took control of the state Senate in 2011. In recent years the population of deep blue northern Virginia has surged, so Obama has a better chance here than Virginia's decades of Republican triumph would indicate.

North Carolina

North Carolina's congressional representation could look more Republican after 2012. The GOP had its first chance in more than a century to redistrict when it took control of the legislature in 2010-a sea change most pronounced in North Carolina but replicated in states across the country where Republicans took control of legislatures.

Florida

Florida will play a large role in the GOP nomination process through its primary on Jan. 31 and its hosting of the GOP convention in Tampa in August. It will also be a key swing state in the general election. The state's large senior population will be responding to Republican proposals on Social Security and Medicare.

Massachusetts

Massachusetts is the rare state where Republicans could lose a Senate seat. Republican Sen. Scott Brown won a huge upset in 2009 to fill the late Sen. Edward Kennedy's seat, but Brown faces a tough candidate this time around: Elizabeth Warren, who headed Obama's newly created Consumer Financial Protection Bureau until recently, currently leads Brown in polls.

Nevada

Nevada, a swing state that President Obama won in 2008, is still feeling the full blast of the recession and the housing crisis. Its foreclosure rate was the worst in the nation in 2009, and remained the worst in 2011. The state's unemployment rate increased last year to 13.4 percent, according to the Bureau of Labor Statistics.

Colorado

Colorado is one swing state with more registered independents than either Republicans or Democrats. President Obama will need to win the support of most independent voters to counter conservative voter excitement; a recent Gallup poll found that Democratic enthusiasm is at its lowest point in a decade, trailing Republican enthusiasm by 27 points.

Missouri

Of the 33 Senate seats up for a vote in 2012, Democrats have 11 more seats to defend than Republicans, and the Republicans need a net gain of four seats to take the Senate majority. Missouri Sen. Claire McCaskill is one of the more vulnerable Democrats.