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Even independents are “more partisan in their behavior” these days, Mr. Greenberg says.

Republican pollster John McLaughlin, however, says there’s still plenty of room for volatility in voters’ choices, with the debates yet to come and the race especially close in certain states.

He says that to get on track after recent distractions, Mr. Romney’s message to undecided voters must be a forward-looking economic pitch — not just that people aren’t better off after the past four years, but that the economy will be much better off after four years under Mr. Romney.

Overall, the race is neck-and-neck in the AP-GfK poll, with 47 percent of likely voters supporting Mr. Obama and 46 percent Mr. Romney.