Romney, a Republican former Massachusetts governor, garnered the support of 49 percent of registered voters in the hypothetical rematch, while Obama, a Democrat, took 45 percent, almost a mirror-image reversal of Romney's popular-vote loss of 51 percent to 47 percent last November, the poll showed.
Discounting the poll's 3.5-point margin of error, the 8-point popular-vote shift, spread across the nation, would have netted Romney wins in 10 additional states, giving him 125 additional electoral votes, meaning he would have won the presidential race with 331 electoral votes to Obama's 207.
Obama won Nov. 6, 2012, with 332 electoral votes to Romney's 206.
Obama's approval rating was also down in the Post-ABC poll to a new low of 42 percent, a fall of 6 points from a month ago. Fifty-five percent of respondents said they disapproved of Obama's job as president.
Both results come amid growing political attacks on Obama for his signature healthcare law. The attacks focus on the broken HealthCare.gov website and on what critics call Obama's broken promise to let people who like their existing insurance plans keep them.
Seeking to quell a growing furor, Obama announced a policy reversal last week that would let people keep individual and small-group policies that would have ended Dec. 31 through September 2015.
The Post-ABC phone poll was conducted among a random national sample of 1,006 adults from Thursday to Sunday.