By Doina Chiacu WASHINGTON (Reuters) – Republican presidential candidates Marco Rubio and Ted Cruz are locked in a battle to become their party's alternative to New York billionaire Donald Trump in Nevada's Tuesday caucus, the last presidential contest before the busy voting month of March. Opinion polls show the two U.S. senators running close in the western state after a Saturday primary in South Carolina where Rubio bested Cruz by fewer than 1,000 votes, Trump swept the state by 10 percentage points and one-time favorite Jeb Bush dropped out. Both Rubio and Cruz came out of South Carolina with sharper criticism of Trump and an eye to the 12 Super Tuesday races on March 1.