Immigration reform's potency as a GOP incumbent slayer will be put to the test in South Carolina, where Sen. Lindsey Graham faces three primary challenges and probably a fourth (The Hill). If he blows them away, as currently seems possible, it could embolden Republicans to support legislation that gives illegal immigrants a path to citizenship. But if his race gets tough, GOP waverers would probably duck for cover, burying the chances of bipartisan reform. Graham is mistrusted by many conservatives because he has for years worked with Democrats, notably on the 2008 bank bailout and climate change legislation, and to help President Obama secure his nominees to the Supreme Court. But immigration may be even more critical to the party base.