Is Obama's post-convention bounce over?

President Obama surged in polls following the Democratic convention two weeks ago, but the nation's two longest-running tracking polls suggest the bump is gone. Gallup and Rasmussen Reports surveys suggest that, nationally, Obama and Mitt Romney are roughly back where they were before the two parties' conventions — essentially neck-and-neck. Still, there are several polls that show Obama has made gains in critical swing states and on nearly every issue. Has the president's bounce faded, or in the post-convention sprint toward election day, has he emerged as the race's solid favorite? Here, a closer look at what the numbers mean:

What do the polls say?

Gallup's latest tracking poll, released Wednesday, has Obama ahead by just one percentage point, with 47 percent to Romney's 46 percent. That's the same lead Obama had, according to Gallup, before the conventions, and it's down from a solid seven-point edge just six days earlier. Rasmussen shows Romney now ahead by two points. An Associated Press/GfK poll also gave Obama a narrow one-point lead, but other polls show him farther ahead: An NBC/Wall Street Journal survey of likely voters, released Tuesday, had Obama up by five percentage points, while a poll released Wednesday by the Pew Research Center gave Obama a commanding eight-percentage-point lead over Romney among likely voters.

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Why the big differences?

All kinds of variables can skew polls one way or another. The sample of voters might include too many Democrats or Republicans, for example. Obama sometimes fares better among registered voters, while the race is tighter among "likely" voters. "Robopolls" might tilt toward one candidate, and those using live interviews the other. The big picture, however, reveals one variable that helps explain the polling disparities — cell phones. On average, says Nate Silver at The New York Times, the surveys that throw them into the mix give Obama a four-percentage-point lead. Those that rely solely on land lines — Rasmussen, for example — paint a rosier picture for Romney, with Obama ahead by just 1.5 points.

What makes cell phones so important?

A third of American households don't have a land line, and rely solely on cell phones, says Silver. Significantly, "potential voters who rely on cell phones belong to more Democratic-leaning demographic groups than those which don't" — younger voters, for example. Excluding these smartphone-toting voters "can bias the results against Democrats," even if, statistically, the demographic groups they belong to are adequately represented. It's unclear whether this is a reflection of the way these voters think, or simply the differences between the polls that include them and those that don't. "Robopolls," for example, didn't show a big convention bounce for Obama and, since federal law prohibits most automated calls to cell phones, they are more likely to use land lines only.

So... has Obama's post-convention bounce faded, or not?

What matters is the trend, says Andrew Malcolm at Investor's Business Daily, so it's pretty clear that bounce is fading. So "forget the Republican doom and gloom drumbeat peddled" in media outlets that are cheerleading for Obama. The president's bounce nationwide does appear to be "eroding," says Jay Bookman at The Atlanta Journal-Constitution. Still, the new picture looks bad for Romney. "In the swing states that he has to have — Ohio, Virginia, Florida — poll after poll shows his situation deteriorating significantly." With Obama cementing his swing-state edge, say Alexander Burns and Emily Schultheis at Politico, Romney needs a big shift in momentum, but things aren't going his way. If anything, says Russ Britt at MarketWatch, Romney's recent stumbles, including his insulting comments about the "47 percent" of Americans who don't pay federal income taxes, could give Obama a brand new bounce when the next round of polls comes out.

Sources: The Atlanta Journal-Constitution, Investor's Business Daily, MarketWatch, The New York Times (2,) Pew Research Center, Politico, The Washington Post

Read more political coverage at The Week's 2012 Election Center.