The Republican Party has won control of the Senate in the US mid-term elections, increasing its power in the final years of Barack Obama’s presidency.
The party took Arkansas, Colorado, Iowa, Montana, North Carolina, South Dakota and West Virginia.
Republican Senate leader Mitch McConnell will lead the chamber.
The party is also set to strengthen its majority in the lower House of Representatives.
Throughout the campaign, Republicans focused on voter dissatisfaction with Mr Obama, describing the vote as a referendum on his presidency.
As the first results came in late on Tuesday, it became clear they had made convincing gains in the roughly one-third of the 100 Senate seats up for election.
With the votes still being counted in the western states, the Republican Party appears easily to have won the six seats it needed to win control of the Senate.
In addition to seats the party won from the Democrats, the Republicans retained seats in Alabama, Georgia, Kansas, Maine, Mississippi, Nebraska, Oklahoma, South Carolina and Tennessee.
McConnell will now become the Senate majority leader, giving him control over the chamber’s legislative agenda and floor proceedings.
“I’ve heard your concerns, I’ve made them my own, you will be heard in Washington,” he said as he declared victory.
“When you get right down to it that’s what this campaign was really all about. It’s wasn’t about me or my opponent. It was about a government that people can no longer trust.”
As well as in the Senate races, the Republicans had a strong showing in the election for the 435 members of the House of Representatives, where they are projected to increase their majority.
They also made gains in the thirty-six governorships up for re-election.
Republican governors survived tough re-election battles in Florida and Wisconsin.
In two results that illustrate the depth of Republican gains, the party’s candidates won in Maryland and Massachusetts, two of the most Democratic-friendly states in the nation.
This is the first time the Republicans have won control of both chambers in Congress since 2006.
They now have the power to complicate, if not block completely, Obama’s agenda in the last two years of his term.
With control of the Senate, the party can also stymie his ability to name new federal judges, cabinet members and senior government officials.
Political gridlock in Congress has already reached historical levels and was a major concern among those voting, with many expressing their frustration with the lack of progress.
Analysts say the Republicans’ victory could make the situation even worse before the president poll in 2016.
Republican leaders have already pledged to move forward on their key policy priorities, pressing Mr Obama to negotiate on their terms
“It’s time for government to start getting results and implementing solutions to the challenges facing our country, starting with our still-struggling economy,” said Republican House Speaker John Boehner.