By Jonathan Beale
BBC News, Washington
In Washington, the new Class of 2007 is being sworn in at the Capitol, and for the first time in more than a decade the Democrats are in control of both houses of Congress.
It is a major shift in power on Capitol Hill that could now challenge the authority of the White House.
Mr Bush has urged his Democratic rivals to work with the White House
As the 110th Congress prepared to take the oath of office, President George W Bush made an appeal for congressional unity.
"We've all been entrusted with public office at a momentous time in our nation's history and together we have important things to do," President Bush said from the Rose Garden on Wednesday morning.
"It is time to set aside politics and focus on the future," he added.
The potentially lame duck president has already held talks with the most powerful woman in Washington.
Nancy Pelosi - the new leader of the House has published a manifesto for the Democrats' first 100 hours. It includes a promise to raise the minimum wage and to clean out corruption.
Stem cell research
9/11 Commission recommendations
Ethics of Congress
Ms Pelosi says her party is ready for the challenge.
"Let it be clear, Democrats are prepared to govern and ready to lead," Ms Pelosi said after the November mid-term elections.
With this shift in power on Capitol Hill the Democrats are not just hoping to influence the domestic agenda, but America's foreign policy too.
After all, many members of the new class were elected because of their opposition to the Iraq war.
Within days, the Democrats are expected to launch a series of investigations into the president's handling of that war.
The new Democrats are moving in - and preparing to lift the lid on the key decisions taken in the White House.
Thomas Mann, a senior fellow at the Brookings Institution, warns that the Democrats alone cannot force a change of course - just make life more difficult for the president.
"Certainly they will make it all the more controversial and make Republicans all the more vulnerable because it's their president who's leading the charge in a way that the public disapproves," says Mr Mann.
The new Speaker, Nancy Pelosi, says the Democrats are ready to lead
The Republicans are in retreat but the Democrats must now prove that they can meet the public's expectations.
Criticism is not enough - they must govern responsibly. The nuclear option of simply cutting off funding for the Iraq war could backfire.
And the euphoria of finally achieving power may be relatively short-lived.