97-year-old Mercedes Rosa Ruiz Mejia from Nicaragua joins over 5,000 other immigrants taking the oath of citizenship during a naturalization ceremony to become new citizens of the U.S. in Los Angeles April 16, 2013. Reuters

Many Americans believe undocumented immigrants are taking jobs from U.S. citizens, according to a poll released Friday. The Rasmussen Reports survey found 51 percent of Americans believe they are competing for jobs against immigrants living in the country without work permits. Less than 40 percent disagreed, and about 11 percent weren't sure, according to the poll.

Economists generally argue that immigration helps the U.S. economy by adding young workers to the workforce. The Brookings Institution’s Michael Greenstone and Adam Looney wrote last year that “on average, immigrant workers increase the opportunities and incomes of Americans" while a study released in January by the liberal Center for American Progress concluded that granting legal status to undocumented workers could create jobs.

But such studies have done little to quell fears about whether undocumented immigrants are taking jobs from U.S. citizens. Republicans were more likely to be worried about undocumented workers than Democrats, the Ramussen survey found. Roughly 71 percent of GOP voters and 51 percent of unaffiliated voters said workers without work permits were taking jobs from American citizens, while 31 percent of Democrats said the same.

U.S. Deportations Over Time | InsideGov

Men were more likely to fear undocumented workers than women. People over the age of 40 also were more worried about competing for jobs against immigrants without work visas compared with younger respondents. Blacks were less afraid of undocumented workers compared with white voters.

Low-income respondents especially felt threatened by undocumented workers, as did Americans without college degrees, the poll found. The poll of 1,000 likely voters was conducted from Aug. 9-10. The poll explained to respondents that the majority of undocumented workers were low-skilled.

Roughly 66 percent of respondents said the government should crackdown on employers who hire immigrants without permission to work in the U.S., the highest figure in nearly four years. The poll also found that voters want the government to more aggressively deport undocumented immigrants.

The U.S. unemployment rate was 5.3 percent in July, or about 8.3 million people. That month, the labor force increased by about 69,000 people.

Most undocumented workers have blue-collar jobs, a recent Pew Research Center report found. But immigrants without work permits are increasingly getting ahead in the U.S. The share of all unauthorized workers with management and professional jobs grew to 13 percent in 2012 from 10 percent in 2007. During the same time, the share of undocumented immigrants with construction or production jobs declined to 29 percent from 34 percent.