The poll, released Wednesday, showed Obama was preferred by 51 percent of likely voters, while 43 percent backed Romney.
The poll was conducted Sept. 12-16, so its results do not reflect voter sentiment following Monday's disclosure of controversial comments Romney made in a talk with donors in Florida, in which he said "there are 47 percent who are with [Obama], who are dependent upon government, who believe that they are victims, who believe the government has a responsibility to care for them, who believe that they are entitled to healthcare, to food, to housing, to you-name-it -- that that's an entitlement -- and the government should give it to them. And they will vote for this president no matter what."
Romney has come in for criticism from conservative Republicans as well as Democrats for the comments but has repeated the argument several times this week.
The Pew poll released Wednesday indicated Obama has a larger September lead than the three most recent nominees who went on to win the presidential election in November, Pew said.
Obama's favorability rating is 55 percent, up from 50 percent in late July, and his unfavorability rating is 42 percent. Romney's favorability rating in the new poll is 45 percent, up from 37 percent in July, and his unfavorability rating is 50 percent.
The Pew poll surveyed 3,019 adults, including 2,424 registered voters. Pew did not report a margin of sampling error but said it would be available on request, and said the survey had a 95 percent level of confidence for different groups participating in the poll.