Underfinanced, thinly organized and reviled in the media, the Trump campaign has nonetheless apparently pulled even in some recent polls with Hillary Clinton. Every pundit can itemize the long list of things that Donald Trump has done all wrong throughout this election season—and yet here he is, poised to overcome all dissent at the Republican convention in Cleveland and to run a competitive race afterward.

Trump’s first and strongest advocate in conservative media, the columnist Ann Coulter, has vividly described the radicalism...