Factbox: Senate races to watch in 2012

Democrats now have 53 seats in the 100-member chamber, including two held by independents who usually caucus with them. Almost half of these seats are being contested at the same time as the presidential election on November 6, 2012, with more than 20 defended by Democrats.

Here are some races that could tip the balance.

HAWAII

Former Governor Linda Lingle's bid for retiring Democratic Senator Daniel Akaka's seat has turned Obama's home turf into an unlikely Senate battleground.

Lingle, Hawaii's first Republican governor in almost 50 years, has appeal among moderate and independent voters. She enjoyed high approval ratings and easily won her second term with 63 percent of the vote in 2006.

The election could pit Lingle against Democratic Representative Mazie Hirono, whom Lingle narrowly beat in the governor's race in 2002. Another candidate is Democrat Ed Case, a former U.S. representative who unsuccessfully challenged Akaka in 2006.

VIRGINIA

Republican George Allen and Democrat Tim Kaine, a pair of former Virginia governors, are the likely contenders for retiring Democratic Senator Jim Webb's seat in a swing state that Democrats will have a tough time winning in the 2012 general election.

Allen is a former U.S. senator who lost his seat to Webb in one of the biggest upsets of the 2006 election cycle. Kaine is a former chairman of the Democratic National Committee.

MASSACHUSETTS

Republicans are eager to hold on to the Senate seat Scott Brown won in 2009 after the death of Democratic Senator Edward Kennedy, a liberal icon.

Democrats are optimistic that Elizabeth Warren, a prominent consumer advocate and Harvard professor known for locking horns with Wall Street, can unseat Brown.

Brown is the first Republican in a generation to win a Senate seat from Massachusetts, a traditionally liberal state.

Recent polls put the two in a dead heat, with Warren appealing to women, middle-to low-income residents and younger voters, and Brown to independents.

MONTANA

Democrat Jon Tester's tough re-election bid reflects troubles the Democratic Party may face in the West.

Polls show Tester and his principal rival for the seat, Republican Representative Denny Rehberg, at a draw.

Both want to paint themselves as the more populist candidate. Rehberg accuses Tester of pandering to Wall Street interests and falling in lock-step with Obama. Tester says Rehberg, a rancher and real-estate developer, is anti-environment and misleading about his wealth.

MISSOURI

Claire McCaskill, a freshman Democrat, faces an uphill battle to hold on to her seat in a Republican-leaning state.

This year she has come under fire for using almost $90,000 in federal money for flights on her private plane and failing to pay $300,000 in local taxes.

Her Republican rivals include Representative Todd Akin and Sarah Steelman, a former state treasurer.

John Brunner, a businessman who chairs a cosmetics and healthcare company, has also entered the Republican race but is relatively unknown in political circles.

NEBRASKA

Ben Nelson is seen as one of the most vulnerable incumbent Democrats in the 2012 election. That is if he even runs. Nelson is expected to announce a decision this month and even members of his staff say they are uncertain what he will do.

Nelson, considered a moderate to conservative lawmaker, is under fire for supporting Obama's healthcare overhaul in a state where Republicans account for 48 percent of all registered voters, according to the Cook Political Report.

Jon Bruning, the state attorney general, was the early Republican favorite, but some question his electability because of his abrasive style.

Other Republican candidates include State Treasurer Don Stenberg and Deb Fischer, a state senator.

NORTH DAKOTA

When former Attorney General Heidi Heitkamp, a Democrat, entered the race for retiring Democratic Senator Kent Conrad's seat she turned what was once thought an easy Republican win into a toss-up.

In a November survey by the Mellman Group for the Democratic Senatorial Campaign Committee, Heitkamp polled ahead of Republican favorite Rick Berg, a representative, at 47 percent to Berg's 42 percent.

But Heitkamp, who supported Obama's health care reforms, could struggle in a state where the president's job approval rating is low.

NEVADA

Republican Senator Dean Heller will defend his seat against Democratic Representative Shelley Berkley.

Heller is a former U.S. representative who was appointed to serve the rest of fellow Republican Jon Ensign's Senate term after Ensign was snared in a sex scandal and resigned earlier this year.

The race could depend on how much each candidate can make inroads outside their base. Berkley has solid backing in her Democratic district which includes Las Vegas and is home to almost 70 percent of Nevadans. Heller's base is in the northern, more rural parts of the state.

NEW MEXICO

Republicans strategists say their party may win in a largely Democratic state because Democrats face a contentious primary to compete for retiring Democratic Senator Jeff Bingaman's seat.

Top Democrats are two-term Representative Martin Heinrich and State Auditor Hector Balderas. Balderas could appeal to the state's more than 40 percent Latino population.

On the Republican side, analysts consider former Representative Heather Wilson, a centrist, as the front-runner although her reputation as a moderate could be an issue.

WISCONSIN

The match-up for retiring Democratic Senator Herb Kohl's seat will likely be between Democratic Representative Tammy Baldwin and former Republican Governor Tommy Thompson.

Thompson, a former U.S. secretary of health and human services, is one of the most popular politicians in the state where he has served four terms as governor. But some conservatives accuse him of being too moderate.

Other contenders for the Republican nomination are Jeff Fitzgerald, a former speaker of the Wisconsin State Assembly who helped push forward controversial legislation on union rights, and former Representative Mark Neumann.

If elected, Baldwin would be the first openly gay U.S. senator.

(Reporting by Lily Kuo in Washington; Editing by Xavier Briand)