-By Nate Silver
Although there are exceptions on either side, like the Gallup national tracking poll, for the most part Mr. Obama seems to be getting stronger results in polls that use live interviewers and that include cellphones in their samples — enough to suggest that he has a clear advantage in the race.
In the polls that use an automated dialing method (“robopolls”) or which exclude cellphones, Mr. Obama’s bounce has been much harder to discern, and the race looks considerably closer.
The difference seems especially pronounced at the state level. Mr. Obama got very strong results in a series of NBC News/Marist College polls last week in Ohio, Florida and Virginia, which included cellphones and used live interviewers. Likewise, Tuesday morning’s series of New York Times / CBS News / Quinnipiac polls had reasonably good news for Mr. Obama in Virginia and Wisconsin.
By contrast, the automated polling firm Rasmussen Reports has recently released polls showing Mr. Obama two points behind Mitt Romney in New Hampshire and three points behind in Colorado — the worst results that it has shown for him in those states all year. Another automated polling firm, Gravis Marketing, recently put Mr. Obama at a five-point deficit in Virginia, in contrast to three traditional polls that put him ahead by margins ranging from four to eight points there. A third automated polling firm, Public Policy Polling, has shown stronger results for Mr. Obama, but they also had him with good results before the conventions, and so haven’t shown him gaining much ground.
These results are consistent with some past research. Roughly one third of American households rely solely on mobile phones and do not have landlines, meaning they will simply be excluded by polls that call landlines only. Potential voters who rely on cellphones belong to more Democratic-leaning demographic groups than those which don’t, and there is reasonably strong empirical evidence that the failure to include them in polls can bias the results against Democrats, even after demographic weightings are applied.