Democrats lose control of US Senate
The Republicans have taken Arkansas, Colorado, Iowa, Montana, North Carolina, South Dakota and West Virginia as they win control of the US Senate.
Republican Senate leader Mitch McConnell will lead the chamber.
The Republican response has been swift, with national chairman Reince Priebus congratulating his candidates.
"The American people have put their trust in the Republican Party, sending a GOP majority to the US Senate... our party's principles and message resonated with voters across the country. This was a rejection of President Obama's failed polices and Harry Reid's dysfunctional Senate."
The party is also set to strengthen its majority in the lower House of Representatives.
As the early results came in late on Tuesday, it became clear the Republicans had made convincing gains in the roughly one-third of the 100 Senate seats up for election.
The party retained seats in Alabama, Georgia, Kansas, Maine, Mississippi, Nebraska, Oklahoma, South Carolina and Tennessee.
One of the key early results came in Kentucky, where Republican Senate Minority Leader McConnell fended off Democrat Alison Lundergan Grimes
Mitch McConnell will now become the Senate majority leader.
"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.
In Florida and Wisconsin, Republican governors have survived tough re-election battles.