Obama leads the Republican former Massachusetts governor 52 percent to 43 percent among all Americans and 51 percent to 45 percent among registered voters, the poll released Sunday night indicated.
The results give Obama his first percentage above 50 percent against Romney among all Americans since July and his first-ever mark above that point among registered voters polled by the Post and ABC.
Obama also leads former House Speaker Newt Gingrich by 15 percentage points overall and 11 points among registered voters, the poll found.
The poll, conducted Wednesday through Saturday, also found Americans say by more than 2 to 1 that the more they learn about Romney, the less they like him.
Judgments about Gingrich, who denounced Romney Saturday night after coming in second behind him in the Nevada caucuses, are about 3 to 1 negative, the poll indicated.
The phone poll of 1,000 adults has a margin of error of 4 percentage points.
Gingrich continued to assail Romney Sunday, attacking him record leading Massachusetts.
"He was pro-abortion, he was pro-gun control, he was pro-tax increase," Gingrich said on NBC's "Meet the Press." "He ended up third from the bottom in job creation, 48,000 manufacturing jobs left because the combination of 'Romneycare' and tax increases made him, in fact, a very weak governor in terms of job creation."
Gingrich called himself the conservative candidate who could offer the biggest contrast with Obama.
He also argued Romney would take a "timid approach" to the economy, while he himself would eliminate the capital gains tax, offer an optional 15 percent flat income tax and permit Social Security savings accounts for young workers.
"The difference between timidly trying to manage at the margins a system that has to be profoundly changed and boldly taking it on is a very, very big difference," Gingrich said. "And I don't think a timid approach is going to beat Obama this fall."
Romney had no comment about Gingrich's assessment.
"Gov. Romney will continue to focus on Obama," Romney spokesman Ryan Williams told The New York Times.
"But we're not going to ignore any candidate. They all have to be taken seriously."
Of Gingrich, he added, "We still do see him as a serious candidate."
All four White House hopefuls face each other Tuesday in Missouri's Republican primary and in Colorado and Minnesota caucuses.
"I think we're going to do very well here in Minnesota. I think we're going to do very well in Colorado," Santorum told "Fox News Sunday."
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