Democrats, who will rule Congress after 12 years out in the cold, plan swift challenges to the United States President George W Bush's conduct of the Iraq war and a 100-hour flurry of legislation designed to appeal to middle-class Americans.
For Bush, the new House of Representatives and Senate convening on Thursday at the white-domed Capitol in Washington will create an inhospitable environment during the last two years of his presidency.
For new House Speaker Nancy Pelosi, 66, it was party time as she prepared to officially take over as the first woman in the high-level post.
The Democrat from San Francisco has raised eyebrows with three days of events celebrating her Italian roots, Roman Catholic faith and East Coast childhood.
US media likened the build-up to a coronation and interpreted it as a bid to shift her reputation from a left-leaning Californian towards the political centre.
With Bush's Republican Party reeling from its defeat in November congressional elections, Pelosi's opening speech on Thursday is set to kick off a critical month for US policy in Iraq.
Bush is expected to present a new war strategy in January, most likely before his annual State of the Union speech to Congress on January 23.
He ordered the strategy review after US voters lifted Democrats into control of both chambers for the first time since 1994 on a platform calling for an exit plan from Iraq.
Newly empowered to set the congressional agenda, Democrats planned to begin hearings as early as next week on the administration's decision-making in the war in Iraq - a stage for likely confrontation in the months ahead.
Pelosi immediately took aim at Bush over Saturday's execution of former Iraqi dictator Saddam Hussein, saying "it is not a substitute for an effective strategy that will bring peace to the region and allow the responsible redeployment of US forces".
The centre-left Democrats say they are more attuned to the worries of average Americans than Republicans, who were dogged at the polls by slumping popular support for Bush and the war he launched in 2003.
Bush still holds broad powers to set foreign and military policy. And while the Democrats have a 16-seat edge in the House, they won only a minimal 51-49 majority in the Senate, limiting their room for manoeuvre.
Still, with no easy solution in sight for Iraq, the Democrats have focussed their debut primarily on domestic issues.
First on the agenda is legislation designed to limit the influence of money-wielding lobbyists and interest groups on lawmakers.
Another planned measure Republicans will find hard to oppose is the first increase in the national minimum wage in a decade.
In the first 100 working hours of the new Congress, House Democrats also plan to pass measures to promote stem-cell research for disease cures, expand government programmes for old-age health benefits and student loans, cut Republican tax breaks for oil companies and step up anti-terrorist security.
"From energy independence to national security, 2007 promises to be a very busy year, and by working together, there's no limit to what we can accomplish," said Democratic Senator Evan Bayh.
How much the Democrats achieve depends not only on the Republican opposition and Bush's veto pen, but also on how the 2008 presidential campaign plays out in the US legislature.
Two top likely contenders for the Democratic Party's nomination - former first lady Hillary Clinton and rising star Barack Obama - sit in the Senate.
Pelosi's schedule before her swearing-in as House speaker included stops in the working-class neighbourhood of Baltimore, the city 65 km northeast of Washington where her father was mayor; a Mass at the women's college where she studied; and dinner at the Italian Embassy in Washington.
The festivities met with sarcasm in conservative circles.
"What? No fireworks?" Mike Murphy, a Republican political consultant, told the Washington Post.