While eyes were focused on the race for the White House on Tuesday, the battle for control of the nation's two chambers of Congress is vital to hopes of easing Washington's partisan political gridlock.
Experts are looking to see if the results will push lawmakers away from the current stalemate but despite voter disgust with a 'do-nothing Congress' in the past two years they agree that little change is likely.
Republicans swept back control of the House in 2010 after a backlash to President Barack Obama's signature health care reforms, and have since used their majority to oppose the White House incumbent's legislative plans.
With a dangerous combination of expiring tax breaks and federal spending cuts looming, the US economy could plunge over its so-called 'fiscal cliff' in January, Congress will take center stage after Tuesday's votes are counted.
All new legislation must be passed in identical form by both the Senate and the House before it is signed into law by the president, a fact Obama knows to his cost and which he has blamed for stalling the second half of his term.
Republicans are expected to hold the House after Tuesday's contests, but the tougher question is whether Democrats can do the same in the Senate.
Some 33 of the Senate's 100 seats are up for grabs, with 23 of those being defended by Democrats, giving Republicans a chance -- albeit slim -- of gaining four seats and seizing control of the chamber.
The Republican charge suffered an early setback when they lost a seat in Maine vacated by retiring moderate Olympia Snowe to former governor Angus King, an independent who is expected to side with the Democrats.
The congressional election campaign has centered on several key races, most notably in the state of Massachusetts, where Republican White House nominee Romney was governor between 2003 and 2007.
Republican Senator Scott Brown scored a surprise victory there in the vote that followed the death of Democratic icon Edward Kennedy.
Brown's win triggered Democratic hand-wringing but his opponent on Tuesday, Elizabeth Warren, has been leading recent polls in the liberal state after overcoming a sluggish start to her campaign.
Another state being closely watched is Virginia, neighboring the US capital.
Democrat Jim Webb is retiring and the party is struggling to hold on to his seat in a state where demographic changes have made it a crucial presidential battleground.
The candidates, George Allen (Republican) and Tim Kaine (Democrat), have waged a vicious battle in recent months with the race centering on defense spending -- Virginia is home to the Pentagon and major defense companies -- and abortion.
Both Kaine and Allen are former governors of the state.
Other key Senate seats up for grabs include Wisconsin, Indiana and Montana.
The fiscal cliff that will dominate discussions in Congress between now and Christmas is a major threat to the economy after a protracted but possibly reckless compromise was agreed last year between Democrats and Republicans.
If Congress doesn't agree on how to cut spending over the medium term, the existing deal would force deep, immediate spending cuts on the government from January 1, while raising taxes.
Economists have predicted that the tax rises will crunch household spending, possibly tip the United States back into recession, and risk another US credit rating downgrade from international ratings agencies such as Standard & Poor's.
Regardless of whether Obama or Romney wins the White House, legislators are under pressure to come up with an alternative fiscal plan before the end of the year, leaving little time after Tuesday's elections to fashion a new deal.
If implemented, the existing plan could cut up to 4% from gross domestic product, according to the International Monetary Fund.