Immigration has been the biggest issue in the Republican contest. It almost sank John McCain's campaign earlier this year and has been a weak flank for Rudy Giuliani. Between "sanctuary cities", "amnesty" and one candidate's use of illegal lawn care workers, immigration has been the subject of the campaign's nastiest exchanges and unabashed fear-mongering. Among the front-runners, Romney is positioning himself as the hardliner's choice. But Fred Thompson has been courting the anti-immigration vote hard. So, too, has Ron Paul. Meanwhile, lingering doubts about his stand on immigration could prevent McCain from scoring the third-place finish he's hoping will help catapult him to victory in New Hampshire.