Republicans will take control of the US House of Representatives when the US Congress meets on Wednesday, vowing to cut spending and repeal health reform.
The new Congress is being sworn in two months after mid-term elections which saw President Obama's Democrats suffer heavy losses to the opposition.
Democratic leader Nancy Pelosi will turn over the speaker's gavel to John Boehner, a Midwestern conservative.
A BBC correspondent says the stage is now set for ideological battle.
The Republicans control the House for the first time in more than four years, while the Democrats have only a slim lead in the Senate.
The BBC's Paul Adams, in Washington, says there are tough fights ahead as the president, determined to press ahead with his reform agenda, locks horns with a Republican Party emboldened by its successes in November.
Republican leaders have vowed to slash spending by as much as $100bn, scrap "job-killing" government regulations, overhaul the tax code, crack down on undocumented immigration, cut diplomatic and foreign aid funds, and investigate the administration.
As soon as next week, the Republicans will launch what is being seen as a symbolic move to repeal President Obama's most ambitious legislative effort so far: the reform of America's healthcare system.
The move is expected to pass in the House, but fail in the Senate, but will be followed by a protracted attempt to pick the reform to pieces, our correspondent says.
Add to this a series of bitter debates over spending and how to control the country's budget deficits, and the scene is set for a tempestuous political season, our correspondent adds. BBC News
