Michael O. Leavitt, a former governor of Utah and a top adviser to Mitt Romney’s 2012 campaign, said he believed the window for any Republican candidate to clinch the nomination before the party’s convention in Cleveland this summer was rapidly closing.
Mr. Leavitt, who has not endorsed a candidate in the race, said he had reviewed the delegate allocation rules for every state and concluded that Mr. Trump would have to capture about 45 percent of the popular vote to win a majority of delegates for the convention. Mr. Trump has not approached that threshold in the polls so far, and Mr. Leavitt said no other candidate was likely to do so as long as so many of them remained in the race. “It will be difficult for him to be a breakaway front-runner,” Mr. Leavitt said of Mr. Trump. “There are a lot of candidates that have staying power, whether it’s by living off the land or a ‘super PAC’ or a combination.”