Pelosi, a California liberal, rejected pressure from moderate House Democrats - and even some liberal allies - who said the widespread defeat cried out for new party leadership. Allies said they doubted Pelosi would make the bid unless she felt certain she had the votes from her colleagues, who are proportionately more liberal after many conservative and moderate Democrats lost on Tuesday.
Her decision set off a likely battle for the No. 2 Democratic leadership job, now held by Majority Leader Steny Hoyer of Maryland. The party's third-ranking leader, House Whip Jim Clyburn of South Carolina, said he will try to keep the job, which will become the second in command when Democrats become the minority.
If Clyburn - the House's highest-ranking African American - prevails, Hoyer would be forced out of the leadership ranks for the first time in many years.
Hoyer said he would make a decision after consulting with lawmakers, adding, "I have received an outpouring of support from Democratic colleagues who have told me that I should remain in our party's leadership."
Hoyer is more centrist than Pelosi, and the two have long had a cordial but somewhat wary relationship.
Pelosi, 70, will seek her colleagues' support to become House minority leader when the new Congress convenes in January. That would keep her atop the Democratic House caucus, which will number about 190 members next year. But it would mark a big drop from being speaker, which carries tremendous power to influence legislation and is second only to the vice president in the line of presidential succession.
House members elect their respective party leaders, although the entire House elects the speaker. That post is almost certain to go Rep. John Boehner, R-Ohio, the current minority leader.
"Our work is far from finished," Pelosi said in a letter to colleagues. "As a result of Tuesday's election, the role of Democrats in the 112th Congress will change, but our commitment to serving the American people will not. We have no intention of allowing our great achievements to be rolled back."
Pelosi said many colleagues "have called with their recommendations on how to continue our fight for the middle class, and have encouraged me to run for House Democratic Leader." Her office released letters of support from various members.
"We're in a political storm, but we don't need to adopt an 'any leader in a storm' mentality," said Rep. Jesse Jackson, Jr., D-Ill., in announcing his support for Pelosi.
Dozens of Republican House candidates attacked their Democratic opponents by tying them to Pelosi and suggesting they would do whatever the San Francisco liberal asked.
Several Democratic lawmakers in conservative districts vowed to oppose Pelosi as speaker, but some of them lost all the same.
One who did survive, Rep. Heath Shuler of North Carolina, had said he might challenge Pelosi because the party needs a more moderate leader. Shuler noted that he lost his job as Washington Redskins quarterback in 1997 after the team performed poorly.
As the magnitude of Tuesday's election losses sunk in, even some longtime supporters of Pelosi said she needed to step aside as the party leader.
"I voted for everything she asked me to vote for," said Rep. Albio Sires, D-N.J. "You know, sometimes in this business it's difficult to know when to move on."
"With all the losses that we had with governors and all the redistricting that's going to be done, we don't need the target," Sires said, referring to the once-a-decade House redistricting process about to begin nationwide.
Rep. John Yarmuth, D-Ky., told WHAS-TV in Louisville: "As good a leader as she has been, I don't think she's the right leader to take us forward." He said Hoyer, who lost two leadership races to Pelosi over the years, would be "a perfect spokesman for the Democratic Party in the House."
Among other moderate Democrats, Rep. Joe Donnelly, an Indiana lawmaker who survived Tuesday's Republican rout, said he was "disappointed that Speaker Pelosi is going to seek the position of Minority Leader." North Carolina Rep. Larry Kissell's office said he hopes Pelosi "will change her mind and step aside."
Republican National Committee Chairman Michael Steele expressed delight when told of Pelosi's decision. In a meeting with reporters, Steele put his hands over his head and applauded.
"My breath is taken away by that announcement," he said, grinning.
If Pelosi remains as the Democrats' House leader, it's possible the party will absorb historic election losses without making significant leadership changes. Senate Majority Leader Harry Reid, D-Nev., will keep his post.
Associated Press writers Philip Elliott, Andrew Taylor, David Espo, Henry C. Jackson and Stephen Ohlemacher contributed to this report.