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.
Polling booths opened in the eastern states at 06:00 local time (11:00 GMT).
The Republicans, who already control the House of Representatives, need to gain just six seats to take the Senate.
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%, 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 on Sunday.
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, says a lot of money has been invested in the tightly-fought races, with some estimates suggesting $200m (£125m) was spent alone in the month of October.
Everything is to play for and the country is holding its breath, she adds.
Analysis: Jon Sopel, North America editor
I have spoken to Democrats and Republicans and they all said the same thing – they were sick of the partisan posturing, the gridlock, the inability to work together, the dysfunctional relationship between Congress and White House, between legislature and executive.
All of which begs the question – what difference will it make if the Republicans do take control of the Senate?
Some are predicting that it will be gridlock on steroids. In other words, just like before – only worse.
Others, who’ve clearly read Voltaire’s Candide and based their philosophy on the ever-optimistic Dr Pangloss, think we will go to the sunny uplands of politics.
Will the US mid-term elections change anything?
The Democrats currently hold a five-seat majority in the Senate, meaning the Republicans need only to win six seats to take control.
Rajini Vaidyanathan speaks to voters in the key battleground state of West Virginia
As the Republicans already have a convincing hold over the lower House of Representatives, a win in the Senate would give them the power to shut down Mr Obama’s policies in the last two years of his term.
Aside from Mr Obama’s unpopularity, there is no single issue that dominates this mid-term poll.
Six Senate races to watch
- North Carolina – Kay Hagan v Thom Tillis
- Arkansas – Mark Pryor v Tom Cotton
- Colorado – Mark Udall v Cory Gardner
- Georgia – Michelle Nunn v David Perdue
- Iowa – Joni Ernst v Bruce Braley
- Kansas – Greg Orman v Pat Roberts
Instead voters will be swayed by a broad variety of concerns including the economy, the environment, immigration, foreign policy, abortion and healthcare.
The most competitive Senate races are expected to take place in the states of North Carolina, Arkansas, Colorado, Georgia, Iowa and Kansas.
Across the board, voters have expressed their dissatisfaction with both parties’ inability to co-operate in Congress and the resulting gridlock this has caused.
If the Republicans do win control of the Senate, the day-to-day running of the chamber will become the responsibility of Senate Minority Leader Mitch McConnell, should he win his own tough campaign for re-election.
As the country then shifts its focus to the 2016 presidential election, Mr Obama is likely to find it increasingly hard to operate as his term draws to a close, analysts say.
Source: BBC News