The Texas congressman built a passionate following in 2008, raising more than $35 million but failing to win any caucuses or primaries. At a news conference in Des Moines, he said the country had changed, with "literally millions of more people now concerned about things I talked about four years ago," including excessive government spending.
His announcement came one day after Mississippi Gov. Haley Barbour announced he wouldn't run. It leaves Paul as the most prominent critic of U.S. military involvement in Afghanistan in the Republican field.
Paul, 75, was the Libertarian Party nominee for president in 1988. He faces competition for votes from Gary Johnson, a former two-term New Mexico governor who recently entered the Republican contest on a libertarian platform. He also will be competing with other conservative candidates for tea party support.
Paul, the father of tea party favorite and Kentucky Sen. Rand Paul, promised a final decision soon about a 2012 run, probably before June. He said he entered the exploratory phase in order to qualify for the first debate of the campaign, scheduled for May 5 in South Carolina.
Paul said he was filing papers with the Federal Election Commission but none had been received late Tuesday.