Democrat Nancy Pelosi, the first female speaker of the US House of Representatives, said on Thursday that she will step down as party leader when Republicans take control of the chamber in January.

"I will not seek re-election to Democratic leadership in the next Congress," Pelosi said in an emotional speech on the House floor. "The hour has come for a new generation to lead the Democratic caucus."

The 82-year-old Pelosi's departure from party leadership marks the end of an era in Washington and comes after Republicans secured a slim House majority in last week's mid-term elections. Democrats retained Senate control.

Democratic President Joe Biden hailed Pelosi as a "fierce defender of democracy" and the "most consequential Speaker of the House of Representatives in our history."

"Because of Nancy Pelosi, the lives of millions and millions of Americans are better, even in districts represented by Republicans who voted against her bills and too often vilify her," Biden said in a statement.

"History will also note her fierceness and resolve to protect our democracy from the violent, deadly insurrection of January 6," when supporters of Republican former president Donald Trump attacked the US Capitol, he said.

Elected to Congress in 1987, Pelosi first became speaker in 2007. Known for keeping a tight grip on party ranks, she presided over both impeachments of Trump during her second stint in the role.

Currently second in the presidential line of succession, after Vice President Kamala Harris, Pelosi said last week that a decision on her future would be influenced by the brutal attack on her husband in the run-up to the November 8 mid-terms.

Paul Pelosi, who is also 82, was left hospitalized with serious injuries after an intruder – possibly looking for the speaker – broke into their California home and attacked him with a hammer.

Pelosi said she would continue to represent her San Francisco district in the next Congress and praised Democrats' better-than-expected performance in the mid-term contest.

"Last week, the American people spoke and their voices were raised in defense of liberty, of the rule of law and of democracy itself," she said. "The people stood in the breach and repelled the assault on democracy." (AFP)