Factbox: Key races in the US election

Millions of Americans vote Tuesday to decide whether President Barack Obama's Democrats will hold the Congress or lose at least one of the two chambers to Republicans.

All 435 House of Representatives seats, 37 of the Senate's 100 slots, and governorships in three dozen states are up for grabs.

Republicans need to capture 39 House seats to retake that chamber, and 10 seats to seize the Senate.

Here are some key races to watch:

Nevada, Senate

Democratic Senate Majority Leader Harry Reid, who shepherded much of Obama's ambitious agenda through the Congress, is fighting for his political life against upstart Tea Party conservative Sharron Angle in a race that is too close to call.

Republicans have made defeating Reid a top priority, but he has clawed back from polls showing him down by double digits earlier this year, and Democrats hope his robust get-out-the-vote machine can win the day.

Angle has reliably run ahead of Reid despite a string of gaffes and controversial statements, including incorrectly claiming that strict Islamist Sharia law had taken over in two US cities and telling Hispanic high school students they looked "Asian."

Unseating Reid, a wily veteran politician and key White House ally, would be a major Republican and Tea Party victory.

Illinois, Senate

The fight for Obama's old Senate seat has been a roller-coaster and now hurtles towards an uncertain outcome as Democratic candidate Alexi Giannoulias and Republican Representative Mark Kirk wage political war.

Giannoulias has highlighted Kirk's admission that he embellished his military record, while Kirk has highlighted that regulators seized the bank owned by his rival's family and that the institution lent money to criminals.

California, Governor

Who will replace Hollywood-superstar-turned-"Guvernator" Arnold Schwarzenegger?

Republicans hope it will be Meg Whitman, former boss of online auction site eBay, who has spent an estimated 120 million dollars of her personal fortune on the race.

Democrats have picked Attorney General Jerry Brown, who governed the state from 1975-1983.

Florida, Senate

Republican candidate and Tea Party darling Marco Rubio is the odds-on favorite in this topsy-turvy race in the vote-rich state that ultimately decided the bizarre 2000 US presidential race.

Rubio upset Florida Governor Charlie Crist in the party's primary fight, leading Crist to seek the job as an independent.

Democratic Representative Kendrick Meek rounds out the field.

West Virginia, Senate

Governor Joe Manchin is vastly popular, with approval ratings at 70 percent, but anger at Obama in a state where the president was never popular may scuttle Manchin's bid to replace the late Robert Byrd, who held the seat for 50 years.

Manchin has taken pains to divorce himself from Obama and Washington Democrats, most memorably in a television ad in which he shoulders a rifle and literally shoots a hole in their legislation to battle climate change.

Republican businessman John Raese has warned voters Manchin would be a "rubber stamp" for Obama's agenda.

Pennsylvania, 8th district

In 2006, a Democratic wave helped sweep Democrat Patrick Murphy past Republican incumbent Mike Fitzpatrick to become the first Iraq war veteran elected to the US Congress. They now face a rematch in what some predict will be a Republican wave year.

Murphy voted yes on three of Obama's major legislative initiatives: overhauling US health care, battling climate change, and stimulating the stalled US economy.

Fitzpatrick has accused the Democrat of being in lockstep with Democratic leaders and doing too little to help create jobs.

Alaska, Senate

Republican incumbent Lisa Murkowski, who suffered a shock primary defeat at the hands of Tea Party favorite Joe Miller, is waging a write-in campaign for re-election.

The Murkowski-Miller duel will showcase the relative strengths of the arch-conservative Tea Party movement and establishment Republicans.

Democrat Scott McAdams trails them both.