By Doina Chiacu and Alana Wise WASHINGTON (Reuters) – Republican presidential candidate Marco Rubio received boosts Monday in his drive to become the mainstream Republican alternative to front-runner Donald Trump, with a string of high-profile endorsements and missteps by rival Ted Cruz's campaign. Rubio, who eked out a second-place finish in South Carolina's primary by fewer than 1,000 votes over Cruz on Saturday, racked up endorsements from prominent Republicans including U.S. Senator Orrin Hatch of Utah, Arkansas Governor Asa Hutchinson and former presidential candidate Bob Dole. Rubio and Cruz came out of South Carolina with sharper criticism of Trump, who swept the Southern state with a comfortable margin of victory.