With fewer than seven weeks to Election Day, a divide has developed between public-opinion polls that show President Obama and Mitt Romney neck-and-neck, and those that depict the president ahead.
The Pew Research Center released its latest in-depth look at the race on Wednesday, showing Obama with an 8-point lead over Romney among likely voters, 51 percent to 43 percent. Six percent preferred another candidate or were undecided, according to the poll, conducted Sept. 12-16.
But an AP-GfK poll released early Wednesday — conducted Sept. 13-17 — showed a much closer race among likely voters, with Obama leading by just 1 point, 47 percent to 46 percent.
Despite the apparent differences between these two polls, they are quite similar in some ways. Each shows the president with a significant advantage among registered voters, with Obama leading by 9 points in the Pew poll and by 10 points in the AP-GfK poll.
One challenge for consumers of political-polling data is that most survey houses have different — in some cases proprietary — ways of determining who is a "likely voter." Pew uses what it describes as "a lengthy set of questions" to determine which respondents are likely to cast ballots in the election. Those include questions about how closely respondents follow news about the election, their voting history, and their self-reported intention to vote. The AP poll also uses a battery of questions, including self-reported vote intention and past behavior, the amount of interest respondents have in the campaign, and whether they know where their polling place is located.
Perhaps as a result of these questions — Pew found Democrats had closed what it calls the "engagement gap" — the number of Obama voters making it through Pew's likely-voter screen is greater than those making it through the AP's.
An NBC News/Wall Street Journal poll of likely voters, conducted over the same dates as the Pew poll, found Obama up by 5 points among likely voters, as margin in between the other two polls. But it more closely resembled the Pew poll in the fact that Obama's lead among registered voters was 6 points, close to the likely-voter difference.
The polls are also varying among the wider universe of registered voters. The latest Gallup Daily Tracking poll, conducted over the past seven days, shows Obama just a single point ahead of Romney, 47 percent to 46 percent. Obama led by as many as 7 points in the poll, but he has seen his lead dwindle over the past week.
Still, some trends are emerging. Romney has not led in a single live-caller poll since the Republican convention last month.
The new Pew poll, while it shows the largest lead for Obama among the currently available data, underscores Romney's uphill battle. Obama wins 44 percent of white voters, well above his target for reelection. Among white Catholics, Obama leads 49 percent to 46 percent. Obama also wins a commanding 72 percent of the Hispanic vote.
The Pew poll surveyed 2,268 likely voters, for a margin of error of plus or minus 2.4 percentage points.