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It could also complicate prospects for a comprehensive fix to the nation's outdated immigration system and tar the Republican Party as anti-immigrant at a time when it needs to broaden its support base of Hispanics and Asians, two of the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