Republican candidates won enough seats in Tuesday's U.S. congressional elections to capture control of the Senate.
Democrats had held a 55-seat majority in the Senate, but Republicans picked up six seats with wins in Arkansas, Colorado, Montana, North Carolina, South Dakota and West Virginia.
However, there were several races still to be decided.
In Louisiana, neither candidate won more than 50 percent of the vote. The Senate race between incumbent Democrat Mary Landrieu and Republican challenger Bill Cassidy will go to a December 6 runoff.
Senate Minority Leader Mitch McConnell was reelected in Kentucky to a sixth term, easily beating his Democratic rival, Alison Lundergan Grimes. It was an ugly race, with both sides struggling to outspend the other, and polls showing Grimes leading McConnell as late as last week.
“She earned a lot of votes, and she earned my respect," McConnell said late Tuesday. "It took a lot of guts to take on a race like this. Because of the business we're in, it also meant she'd take some heat. I admire her willingness to step into the arena and fight as hard as she did.”
With Republicans in control, McConnell will become Senate majority leader. He will have the authority to decide which bills to bring up for a vote.
President Barack Obama said Senate Democrats faced what could be the toughest races since 1958, when Republicans lost 13 Senate seats under then-President Dwight Eisenhower.
In the mid-term races for one-third of the 100 senate seats, Republicans needed to gain six seats to wrest control of the Senate from Democrats. Republicans held the majority in the House of Representatives and are expected to increase their numbers.
In other Senate victories for the Republicans, Lindsay Graham was reelected in South Carolina, while a second Republican, Tim Scott, won the election to finish the term of Senator Jim DeMint, who resigned.
Scott became the first African-American elected statewide in South Carolina since the end of the American Civil War.
Republicans also picked up a seat in West Virginia that had been held by Democrats when Representative Shelley Moore Capito won the race to replace retiring Democratic Senator Jay Rockefeller.
Former South Dakota Governor Mike Rounds, a Republican, will take over from retiring senator Tim Johnson. Rounds held off Democrat Rick Wieland and two independents.
Republican Cory Gardner defeated Colorado's incumbent Democratic Senator Mark Udall.
Republican Tom Cotton won a bitterly contested Senate race in Arkansas. TV network projections gave Cotton a victory over two-time Democratic Senator Mark Pryor.
But the Democrats also secured a big win for an incumbent Tuesday as Democrat Jeanne Shaheen won re-election to the U.S. Senate from New Hampshire on Tuesday, beating back an aggressive campaign from Republican Scott Brown.
With control of Congress, Republicans could raise new disputes with Obama over his signature legislative achievement, health care reform, which has allowed millions of people to secure insurance coverage they could not previously afford.
Many Republicans view it as excessive government involvement in people's health care and call for repeal of the law.
Attacks on Obama
Republicans also have attacked Obama's handling of the Ebola crisis and called for approval of an oil pipeline from Canada through the central U.S., and a curb on government regulation of businesses.
Some opposition lawmakers have also disputed the president's handling of Russia's intervention in Ukraine and U.S. airstrikes against Islamic State militants in Iraq and Syria.
In the United States, the two main political parties are feuding over spending and tax policies and immigration reforms.
Obama has vowed to set new immigration rules by executive order by the end of the year, after the House did not act on comprehensive reforms approved by the Senate.
Some Republicans already are saying they will seek to block the president from unilaterally changing the country's immigration policies to allow millions of migrants who entered illegally to stay in the United States.
VOA's Mike O'Sullivan contributed to this articled from Washington.