Like a fever or an irregular heartbeat, Donald Trump’s surge in the GOP primary polls provides important information about the state of the party of Lincoln. Mr. Trump, a terrible candidate in many ways, is inexperienced as a politician. Rhetorically, his mode tends to excess, and only to excess. Moreover, as some have pointed out, given the positions Trump has held over the years, calling him a “Republican” or a “conservative” is a stretch. That said, his current positions are conservative. Which is why his candidacy—in or out of the GOP—might harm the party’s chances of retaking the presidency, and this, in addition to his drawing so much attention away from better candidates, has angered many commentators.