He praised all of his Democratic rivals but endorsed no one. He encouraged voters to "take a long and thoughtful look" at all of them.
Richardson said that although his support at the polls lagged the front-runners, many of his leading rivals had moved closer to his positions on such issues as the war in Iraq and educating young Americans at home.
On a less serious note, he estimated the long campaign had included 200 debates. He quickly amended that 24 but said "it felt like 200."
Richardson, 60, announced his decision at the state Capitol, saying he was returning to "the best job in the world" as New Mexico's governor.
With the New Mexico Legislature convening for its annual one-month regular session next Tuesday, he said with a grin to all those involved: "I'm back."
Richardson had one of the most wide-ranging resumes of any candidate ever to run for the presidency, bringing experience from his time in Congress, President Clinton's Cabinet as energy secretary, as U.S. ambassador to the United Nations, in the New Mexico Statehouse as well as his unique role as a freelance diplomat. As a Hispanic man, he added to the unprecedented diversity in the Democratic field that also included a woman and a black man.
But Hillary Rodham Clinton and Barack Obama dominated the spotlight in the campaign, and Richardson was never able to become a top-tier contender, trailing well behind them and former North Carolina Sen. John Edwards.
Richardson fell below 5 percent in the New Hampshire primary Tuesday and came in with just 2 percent in the Iowa caucuses last week.
Edwards congratulated Richardson, saying he had run a good race.
There was no immediate comment from the other campaigns.
"He was a very good candidate, a serious candidate," Edwards said in Columbia, S.C. "I congratulate him. He ought to be proud of what he's done. What's happened is, over time the race is becoming more focused. I think that's good for democracy. I think this thing's going on for a long time."
Richardson was easily elected to two terms as governor but will be forced from office by term limits in 2010.
