"After much prayer and serious consideration, I have decided that I will not be seeking the 2012 GOP nomination for President of the United States," Palin said in a letter carried by US media.
"I believe that at this time I can be more effective in a decisive role to help elect other true public servants to office -- from the nation's governors to congressional seats and the presidency," she wrote.
"In the coming weeks I will help coordinate strategies to assist in replacing the president, re-taking the Senate, and maintaining the House."
Palin, a darling of the ultra-conservative Tea Party movement who was Senator John McCain's surprise running mate in 2008, had tantalized supporters for months, launching a bus tour with stops in crucial early-voting states.
But her showing in the opinion polls never really lived up to the media hype. And independent observers didn't believe she could make a credible run and unseat President Barack Obama in next year's November election.
Her decision came the day after New Jersey Governor Chris Christie also disappointed those wanting a shake-up of the Republican field and leaves an expected two-way battle for the nomination between former Massachusetts governor Mitt Romney and Texas Governor Rick Perry.
With Obama vulnerable due to the faltering economy and stubbornly high unemployment, Republicans are desperate to find a candidate who can rally the conservative base but who also doesn't turn off independents.
Christie's and Palin's decisions will disappoint those with lingering doubts about Romney, the current front-runner. Many US Christians view his Mormon faith as odd, a cult or even a heresy and others see the former Massachusetts governor as an inauthentic conservative.
Perry, hailed by the right as the savior of the race when he jumped in on August 13, has slipped in the polls after failing to sparkle in early debates despite his strong conservative credentials.
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