Washington, D.C ? Watching the polls to figure out who’s up and who’s down this election season? Be careful. The poll may have a pro-Republican bias.

The ranks of Americans who use only cell phones have skyrocketed. Some public polls don’t survey them, however, and they miss a group of people who are more likely to vote Democratic, including the young, the poor, Hispanics and African-Americans.

The nonpartisan Pew Research Center recently found that in four out of five national polls this year, polls that contact only those with land-line phones gave Republicans a 4- to 6-percentage-point edge over Democrats, compared with polls that included cell phones.

In the most recent poll, a survey of likely voters reached via land lines gave Republicans a 12-point edge, 53-41 percent. Polls that also called voters who only use cell phones found the Republican edge was 7 points, 50-43 percent.

“Cell-only adults are demographically and politically different than those who live in land-line households,” the Pew report said. “As a result, election polls that rely only on land-line samples may be biased.”

The challenge in measuring public opinion has grown as more Americans rely on cell phones. In four and half years, the percentage of Americans 18 and older who rely only on cell phones has skyrocketed from 9.6 percent to 22.9 percent, according to the National Center for Health Statistics.

Similarly, the tally of households with cell phones only has risen from 10.5 percent to 24.5 percent. Another 14.9 percent of homes have land lines but report receiving most or all calls via cell phones.