Republican 2016 presidential hopefuls Scott Walker and Rick Santorum are suggesting a potentially controversial way to boost US citizens' job prospects: admit fewer legal immigrants into the United States.

The notion, absent from presidential politics for at least 20 years, could help them tap into the frustrations of working-class voters who have struggled with stagnant wages and reduced job opportunities since the economic crisis of 2007-09.

It could also complicate prospects for a comprehensive fix to the nation's outdated immigration system and tar the Republican Party as anti-immigrant at a time when it needs to broaden its support base of Hispanics and Asians, two of the biggest groups of legal immigrants in the United States.

"This hurts our efforts. I think people need to tone down the rhetoric," said Hugo Chavez-Rey, chairman of a Hispanic Republican group in the battleground state of Colorado.

Since 1989, the United States has been letting in about 1 million new immigrants per year, a level comparable to the last great wave of European immigration at the turn of the 20th century. The US Census Bureau estimates there are now 43.3 million foreign-born residents in the United States, and within 10 years immigrants will account for 15 percent of the population, a record high.

Vocal champions

About 2 in 5 US citizens think those levels are too high, according to polling by Gallup.

Many Republican presidential candidates are vocal champions of legal immigration. Former Florida Governor Jeb Bush, the front-runner in the Republican race, says more legal immigrants are needed to boost economic growth, while South Carolina Senator Lindsey Graham says they can help care for an aging population.

Kentucky Senator Rand Paul has said higher levels of legal immigration would lead to lower levels of illegal immigration, while former Arkansas Governor Mike Huckabee said last month, "Let's get as many people here as want to come." Florida Senator Marco Rubio and Texas Senator Ted Cruz frequently invoke their Cuban-immigrant parents in stump speeches, and both have called for expanding guest-worker programs.

Few national politicians have called for limiting legal immigration since the 1990s, when Republican candidate Pat Buchanan warned that immigrants would erode the influence of white US citizens.

That argument still appeals to voters like Colorado retiree Jan Herron, who see the nation's increasing diversity as a threat to their way of life.

"California is gone because of the invasion," Herron said, referring to that state's growing Hispanic population. "The same thing is happening here in Colorado."