As he led a crowd of picnicking families in the “YMCA” dance, Trent Staggs gleefully waved a “Utah for Trump” flag at a recent campaign rally — one more not-so-subtle reminder to voters that he is backed by the former president in the race to replace retiring U.S. Sen. Mitt Romney. That endorsement propelled Staggs, who was little known outside the Salt Lake City suburb he leads, to victory at the state's Republican convention in April, where delegates lean far-right. But his credentials with Donald Trump's “Make America Great Again” movement may not be enough to win Tuesday's primary, when...