As House Democrats push forward with an impeachment inquiry into President Trump, Republicans have largely rushed to Mr. Trump’s defense, or at least tempered their criticism to avoid his furious reprisals. Among the handful of exceptions, though, there has been none louder or more prominent than Senator Mitt Romney of Utah, a figure who once embodied the essence of the Republican Party before Mr. Trump commandeered it, and is now in a lonely category of his own. Since the first reports a week ago that Mr. Trump urged President Volodymyr Zelensky of Ukraine to investigate former Vice President Joseph R....