With the votes still being counted in many states, the Republican Party easily won the six seats it needed to win control of the Senate.
The party now controls 52 seats, and is tipped to win more.
As well as taking the Senate, the Republicans are projected to increase their majority in the House of Representatives to levels not seen since before World War Two.
They also made gains among the 36 governorships up for re-election.
When the new Congress is sworn in in January, it will mark the first time the Republicans have held both chambers since 2006.
They will now have the power to complicate, if not block completely, Mr Obama's agenda in the last two years of his tenure in the White House.
Control of the Senate will also enable the Republicans to stymie his ability to name new federal judges, cabinet members and senior government officials.
With the votes still being counted in many states, the Republican Party easily won the six seats it needed to win control of the Senate.
The party now controls 52 seats, and is tipped to win more.
As well as taking the Senate, the Republicans are projected to increase their majority in the House of Representatives to levels not seen since before World War Two.
They also made gains among the 36 governorships up for re-election.
When the new Congress is sworn in in January, it will mark the first time the Republicans have held both chambers since 2006.
They will now have the power to complicate, if not block completely, Mr Obama's agenda in the last two years of his tenure in the White House.
Control of the Senate will also enable the Republicans to stymie his ability to name new federal judges, cabinet members and senior government officials.
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