Plan B includes formation of TheCainSolutions.com, which he described as a grassroots effort to bring government back to the people. It would also continue to push his signature 9-9-9 plan.
Cain's announcement was a remarkable turnabout for a man that just weeks ago vaulted out of nowhere to the top of the GOP field, propelled by a populist, outsider appeal and his tax overhaul plan.
Saturday's event was a bizarre piece of political theater even for a campaign that has seemed to thrive on defying convention.
Cain marked the end of his bid at what was supposed to be the grand opening of his new campaign headquarters in Atlanta. Minutes before he took the stage to pull the plug, aides and supporters took to the podium to urge attendees to vote for Cain and travel to early voting states to rev up support for his bid.
"Join the Cain train," David McCleary, Cain's Georgia director, urged the audience.
Volunteers had been up through the night preparing the former flooring warehouse to open as the new hub of Cain's early-state outreach.
He marveled at rising from a childhood in Atlanta marked by segregated water fountains and poverty to what he called "the final four" of the presidential contest.
The former Godfather's Pizza chief executive, who has never held elective office, rose just weeks ago to lead the Republican race. But he fumbled policy questions, leaving some to wonder whether he was ready for the presidency. Then it was revealed at the end of October that the National Restaurant Association had paid settlements to two women who claimed Cain sexually harassed them while he was president of the organization.
A third woman told The Associated Press that Cain made inappropriate sexual advances but that she didn't file a complaint. A fourth woman also stepped forward to accuse Cain of groping her in a car in 1997.
Cain has denied wrongdoing in all cases and continued to do so Saturday.
Polls suggest his popularity had suffered. A Des Moines Register poll released Friday showed Cain's support plunging, with backing from 8 percent of Republican caucus goers in Iowa, compared with 23 percent a month ago.
Follow Shannon McCaffrey: http://www.twitter.com/smccaffrey13
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