With little time to go before Michigan’s Republican U.S. Senate primary Tuesday, signs are mounting that Clark Durant — former Reagan administration official and lifelong movement conservative — may overtake the longtime front-runner in the polls, former Rep. Pete Hoekstra. Should Durant win the primary to oppose Democratic Sen. Debbie Stabenow will be “Step Four” in a string of pitched battles in GOP Senate contests won by tea party-backed conservatives against establishment candidates. In Indiana, State Treasurer Richard Mourdock defeated longtime Sen. Dick Lugar for renomination and in Nebraska, State Sen. Deb Fischer emerged atop two better-known officials. Most recently,...