Americans are voting in mid-term elections which will decide who controls the Senate and pave the way for the 2016 race for the White House.
The Republicans, who already control the House of Representatives, need to gain just six seats to take the Senate, the BBC reported.
Meanwhile the Democrats are battling to stay ahead as President Barack Obama's approval ratings fall to the lowest they have been since he was elected.
Many analysts predict a Republican victory as Mr Obama's popularity rate has failed to climb much above 40 percent, despite recent improvements in the economy.
"This is a referendum on the president," Republican senator and potential 2016 presidential candidate Rand Paul told NBC's Meet the Press at the weekend.
But Democrats say their proven ability to rally their supporters ahead of elections could still give them the advantage.
"Grab everybody you know, get them out to vote, don't stay home, don't let somebody else choose your future for you," Mr Obama said during a campaign rally.
Without the focus of a presidential campaign, the mid-terms - which are named because they fall in the middle of a presidential term - typically see a low voter turnout.
They also typically favour the party that is not in power.
This year, a little over a third of the 100-seat Senate, all 435 members of the House of Representatives, 36 out of 50 state governors, and countless state and local offices are up for election.
The most closely watched action will be the races that will determine control of the Senate, the upper chamber of Congress.
The BBC's Jane O'Brien in Arlington, Virginia, said a lot of money had been invested in the tightly-fought races, with some estimates suggesting $US200m was spent alone in the month of October.
Everything is to play for and the country is holding its breath, she adds.