For Republicans, the presidential primary contest has become a nightmare from which they can’t wake up. Their front-running candidate cannot close the deal; their runners-up cannot surge sufficiently to displace the front-runner. Each candidate’s favorability rating has drooped as the campaign rolls on. None has been able to broaden his support beyond a relatively narrow base. The prospect that no candidate will amass a decisive majority of delegates well before the party’s August convention looks increasingly plausible.