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Many polls continue to collect their data from calls to land lines rather than cellphones, which could increase responses from older voters who rely on their home phones and minimize participation among younger voters.

Mr. Rasmussen predicts that polling might move entirely online in five to 10 years.

The different methods and varieties of approaches among the various pollsters mean poll watchers need to take individual surveys with a grain of salt.

“Look across all of the polling,” Mr. Franklin said. “Don’t just try to believe that one poll is always right or that one is always the best. If you see all of the polls pointing in the same direction, that’s certainly more reliable.”