"The judgment of the voters is now clear," Lieberman said. "For me, it is now time to make a difficult but realistic decision. I have decided tonight to end my quest for the presidency of the United States of America."
He was the fourth to drop out.
Steve Murphy, former campaign manager for Missouri Representative Dick Gephardt, who quit after the Iowa caucuses, said the race for the nomination was all but settled in his view.
"The only question is who is going to be the last man standing" against Kerry, he said.
Edwards and Clark had hoped to make progress on that question, at least, but it was left unsettled by the split decision in the two states not won by Kerry.
Clark had spent US$11 million on TV ads in trying to climb out of the pack; Edwards fell just short in Oklahoma of showing he could win outside the South, trailing Clark by less than 1,000 votes.
Dean saved his money for a last stand in Wisconsin on Feb. 17, a long-shot strategy that some of his own advisers questioned.
"We're going to have a tough night," Dean told supporters as he promised to keep "going and going and going and going and going -- just like the Energizer bunny."
Dean, a former Vermont governor, ran out of cash and momentum after finishing third in Iowa and a distant second in New Hampshire. He ran no TV ads in the seven states at stake on Tuesday and intended to stay off the air for a spate of other contests until the Wisconsin vote.
Kerry is racking up endorsements as well as delegates. The 1.2 million-member American Federation of Teachers, the country's second largest teachers' union, planned to come out for Kerry on yesterday, a senior union official said on condition of anonymity.
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