Washington (CNN) – As Sen. John McCain readies for a potential primary challenge from the right this fall, the Arizona Republican is playing his trump card. Sarah Palin, McCain's former running mate and perhaps the most powerful brand in Republican politics, will stump for McCain in Arizona on March 26 and 27, a Palin adviser told CNN. The presence of Palin - and the considerable crowds and media attention she will draw - might give pause to J.D. Hayworth, the former Republican congressman and radio host who opposes McCain's position on illegal immigration and is mulling a Senate run of...