FACTBOX: Key 2010 Senate races to watch

WASHINGTON (Reuters) - Following are some of the 2010 U.S. Senate races that are seen as the most competitive:

Arkansas: Incumbent Democratic Senator Blanche Lincoln is facing a tough re-election in a state that voted Republican in the 2008 presidential race. Well into her second six-year term, critics say she has failed to make an impression as a senator.

Connecticut: Incumbent Democratic Senator Christopher Dodd was expected to survive his re-election bid, but his popularity has plummeted since it was revealed he inserted language into a $787 billion economic stimulus package that paved the way for $165 million in retention bonuses to be paid to employees of American International Group Inc. The embattled financial firm is being bailed out by the government.

Florida: Republican Senator Mel Martinez is retiring, and both parties see this race as a bellwether given its history of close elections. The state has been hit particularly hard by the housing market collapse and Cuban-Americans may also show their anger at President Barack Obama's decision to loosen travel and financial restrictions on Cuba. Republicans are waiting to see if Governor Charlie Crist will run for them and Democrats are seeing several candidates consider a bid.

Missouri: Republican Senator Christopher "Kit" Bond is retiring in a state that Obama narrowly in 2008. The contest will be fierce with Representative Roy Blunt, once part of the House Republican leadership, running against Robin Carnahan, Missouri's secretary of state and daughter of former Governor

Mel Carnahan, who died in a plane crash in 2000 shortly before winning a U.S. Senate seat.

Ohio: Republican Senator George Voinovich is retiring. The state has been particularly hard hit by the recession and supported Obama in 2008. Rob Portman, who used to run the White House Budget Office and was the U.S. Trade Representative under President George W. Bush is seeking the seat, while two Democratic state officials are eyeing bids.

Pennsylvania: Republican Senator Arlen Specter is running for another term and is facing a fierce challenge within his own party. Pat Toomey, who narrowly lost to Specter in a primary contest in 2004, is challenging him again. Several Democrats are considering a run in the state, which Obama carried last year, including a few House lawmakers and Joseph Torsella, who lost a Democratic primary for a House of Representatives seat in 2004.

Other potentially vulnerable seats include those of Democratic Senate Majority Harry Reid of Nevada, Republican Senator Jim Bunning of Kentucky, New York Democratic Senator Kirsten Gillibrand, Louisiana Republican Senator David Vitter and retiring Republican Senator Judd Gregg.

(Reporting by Jeremy Pelofsky, editing by Paul Simao)