WASHINGTON — In a day of ceremony and historic change, Rep. Nancy Pelosi became the nation’s first female House speaker yesterday as Democrats eagerly took control of Congress for the final two years of President Bush’s term.

“The Democrats are back,” Ms. Pelosi said, and she immediately set the rank-and-file to work passing tougher ethics rules.

As is customary, the opening moments of the 110th Congress produced pledges of bipartisanship at both ends of the Capitol. Yet Ms. Pelosi and the Senate majority leader, Senator Reid, signaled a new political order when they swiftly challenged Mr. Bush over the war in Iraq.

“No issue in our country is more important than finding an end to this intractable war,” Mr. Reid, a Democrat of Nevada, said. “Completing the mission in Iraq is the president’s job, and we will do everything in our power to ensure he fulfills it.” Mr. Bush is expected to announce a revised strategy next week for the war, which has claimed the lives of more than 3,000 members of the American armed forces.

The two houses convened at the stroke of noon, under Democratic control together for the first time since 1994.

That meant a return to power for men long used to wielding it. Lawmakers such as Senator Kennedy of Massachusetts and Rep. John Dingell of Michigan are committee chairmen again, for example.

And it brought reduced circumstances to others — no one more so than Rep. Dennis Hastert, a Republican of Illinois, just a member of the ranks after eight tumultuous years as speaker.

By custom, lawmakers brought their children to opening ceremonies in the House, and Ms. Pelosi swept into the chamber accompanied by her grandchildren.

Formality reigned, as always, in the Senate, where Vice President Cheney administered the oath of office to 33 new and newly reelected senators.

President Clinton watched from the gallery as his wife, Senator Clinton of New York, was sworn in for a second term.

In a chance encounter, he dodged questions about her presidential ambitions. “I would like not to talk about it today,” he said.

It fell to Rep. John Boehner of Ohio, the new House Republican leader, to hand the gavel — symbol of the speaker’s authority — to Ms. Pelosi. “Whether you’re a Republican, Democrat, or an independent, this is a cause for celebration,” he said, noting her place in history.

But he also gave notice to the party she leads, adding, “Republicans will hold the incoming majority accountable for its promises, and its actions.”