The lack of a legal visa category for the jobs most illegal immigrants fill is a core reason for illegal immigration.

FORBES: Too Few Work Visas in New Immigration Bill

Today, there are few legal avenues for lesser-skilled foreign workers, which is a prime reason for illegal immigration.

FORBES: Colbert Hits the Mark in Hill Testimony

One reason appears to be reduced immigration, both legal and illegal, because of diminished job opportunities and more effective border enforcement.

ECONOMIST: Troubling trends behind the falling unemployment rate

" It's one reason that the group -- which advocates both an end to illegal immigration and a dramatic reduction in legal immigration, to pre-1965 levels -- gave Ryan's career in Congress a "C.

CNN: Is Ryan wooing Hispanics for 2016?

If illegal immigration didn't work for Republicans as a campaign issue, Camarota says there's a good reason.

NPR: Immigration Logjam May Break with New Congress