Initial results in the US mid-term elections show the Republican Party is making early gains in the Senate, where they need to win just six seats to take control of the chamber.
The party has already picked up two Senate seats in Arkansas and West Virginia, says the AP news agency.
The Republican Party leader in the Senate, Mitch McConnell, has retained his seat for Kentucky.
But the Democrats have held onto the key Senate seat of New Hampshire.
As polls close across the country, early results indicate a number of both Democratic and Republican Senate incumbents have held onto their seats.
Projections show Republicans have retained seats in Mississippi, Alabama, Maine, South Carolina, Oklahoma and Tennessee.
Democrats have retained seats in Massachusetts, Rhode Island, Delaware and New Jersey.
As well as 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.
Early projections indicate the Republicans will increase their control over the House.
Throughout the campaign, the Democrats have battled to stay ahead as President Barack Obama’s approval ratings fell to the lowest they have been since he was elected.
Many analysts have predicted 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.
If the Republicans succeed in taking control of the Senate from the Democratic Party, Mitch McConnell is set to become the majority leader.
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.
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.