The Senate-will-never-convict club can discount this history, pointing instead to congressional Republicans' sycophancy during the past two years, and credibly say, "GOP senators are in lock-step with the president and will rally around their fellow Republican." And indeed, getting 20 out of 53 Republican senators to agree to boot him from office won't be easy under any circumstances.

Trump, however, is far from an institutional Republican. He'd never ran for office as a Republican before the 2016 presidential election. From the 1980s into the Obama years, he donated more to Democratic candidates than to Republican ones. As late as 2004, he said he identified more as a Democrat than as a Republican.

His party credentials contrast sharply with the most recent nearly impeached Republican president, Richard Nixon. By the time the Judiciary Committee voted on articles of impeachment against Nixon, he had been a steady partisan for almost 30 years: a Republican representative and senator from California from 1947 until 1953, vice president of the United States for eight years under Dwight Eisenhower, the party's standard-bearer in the 1960 presidential election, Republican candidate in the 1962 California gubernatorial race and president since 1969.