By Doina Chiacu and Alana Wise WASHINGTON (Reuters) – Marco Rubio emerged from a razor-thin second-place finish in South Carolina's presidential primary on Sunday with the Republican field narrowed and his target clear: Donald Trump. Rubio squeaked by rival and U.S. Senate colleague Ted Cruz in a vote on Saturday that cemented the appeal of firebrand outsider Trump as well as the failure of a candidate from one of America's political dynasties: Jeb Bush. The five remaining Republican candidates are now scrambling for voters in 12 states that hold nominating contests on March 1, also known as Super Tuesday, the next milestone on the road to the Nov. 8 presidential election.