While eyes were focused on the presidential vote, the battle for control of the nation's two chambers of Congress is vital to chances of making any headway toward 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, there seemed little chance of breaking the political deadlock.
The Republican effort to take the Senate took an early dent in Maine, where former governor Angus King, an independent who is expected to side with the Democrats, was projected to succeed retiring Republican Olympia Snowe.
A second setback followed in Connecticut, where Democrat Christopher Murphy was projected to defeat his well-funded Republican opponent Linda McMahon, picking up the seat of outgoing independent Senator Joe Lieberman.
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 and Congress will take center stage after Tuesday's votes are counted.