By Steve Holland CHARLESTON, S.C. (Reuters) – Republican presidential candidates Marco Rubio and Jeb Bush are locked in a bare-knuckled fight for third place in a South Carolina primary vote on Saturday, afraid that anything less could blunt their White House hopes. With front-runner Donald Trump fighting to hold off U.S. Senator Ted Cruz of Texas and win his second straight early nominating contest, Rubio and Bush are battling to become the main anti-Trump alternative from among establishment Republicans seeking the party's nomination for the Nov. 8 election. An NBC News-Wall Street Journal/Marist poll released on Friday showed Trump in the lead with support from 28 percent of likely Republican primary voters, followed by Cruz at 23 percent.