Republican presidential candidate Mitt Romney, with Sen. John Thune of South Dakota, speaks Wednesday to workers at Nationwide Insurance Co. in Des Moines, Iowa. Mr. Romney is using a tough stance against illegal immigration to separate himself from his rivals for the nomination. (Associated Press)

Republican presidential candidate Mitt Romney, with Sen. John Thune of South Dakota, speaks Wednesday to workers at Nationwide Insurance Co. in Des Moines, Iowa. Mr. Romney is using a tough stance against illegal immigration to separate himself from his rivals for the nomination. (Associated Press)