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

"For me, the hour has come for a new generation to lead the Democratic caucus that I so deeply respect," said the 82-year-old Pelosi, who first became speaker in 2007 and later presided over both impeachments of Donald Trump.

"I will not seek re-election to Democratic leadership in the next Congress," Pelosi told lawmakers in a speech on the House floor.

With congressional control now split and Pelosi unseated as leader in the House, the 82-year-old Californian -- known for keeping a tight grip on party ranks as the first woman speaker --  had previously indicated her time as a lawmaker might be up. . 

Pelosi said last week her final decision -- should Democrats lose the House -- would be influenced by the brutal attack on her elderly husband in the runup to the November 8 midterms.

Paul Pelosi, who is also 82, was hospitalised with serious injuries after an intruder -- possibly looking for the Speaker -- broke into their San Francisco home and attacked him with a hammer. 

On Wednesday Pelosi praised Democrats' better-than-expected performance in the midterm contest, saying the party "defied expectations."

Republicans failed to take control of the Senate, and recaptured the House with a far smaller majority than they had been counting on, in a historically weak performance in the November 8 midterms.

"In the next Congress, House Democrats will continue to play a leading role in supporting President Biden's agenda -- with strong leverage over a scant Republican majority," Pelosi said in a statement.