Pennsylvania: Obama 51, Romney 42

Barely half of poll respondents approved of the job Obama has done as president, a finding that Romney could build on, Geller said. He said he expected the race to tighten.

But only about four in 10 respondents had a favorable view of Romney, who, starting with this week’s Republican National Convention, must burnish a personal image tarnished by a summer’s worth of negative Obama ads.

Romney’s choice of Rep. Paul Ryan of Wisconsin to be his running mate does not appear to have given him a Pennsylvania boost. Plaut said the numbers showed Romney might have done better in the state if he had picked Gov. Christie, who has a strong favorability rating across the Delaware from his home turf.

Plaut said it was significant that 57 percent of poll respondents said they believed Obama would win Pennsylvania and its 20 electoral votes; only 29 percent said they thought Romney would. That expectation could become self-fulfilling if it influences Election Day turnout.