The Russian taint.
Slowly, vast swaths of the American public appear to be concluding that the Trump regime is in some spooky sense beholden to the Russian government. It is a cumulative and piecemeal process, and what is important to some people does not move others. But, overall, the conclusion is hard to avoid--the Russians successfully 'hacked' the 2016 election, Trumps surrogates, agents, campaign manager, etc., are in pretty routine contact with Russians counterparts, Trump himself and his family have business interests that fatally compromise the independence and autonomy of the American presidency vis-a-vis Russia. And that's without even getting to lurid hints at filmed sexual depravity or rumors of a half-billion dollar Russian 'investment' in Trump's businesses.
Maybe it will all blow over. Not likely. Not with the attorney general getting caught lying under oath about it (excuse me, misremembering in his testimony). It is kind of funny watching the party of the Benghazi witch hunt balking at looking into a real problem. The people who wanted to pin personal responsibility for consular security issues on a secretary of state who, I doubt, could have found the consulate if you'd given her a map of Benghazi?
Almost 70 years ago, Republicans in Congress engaged in an earlier witch hunt, seeking Communists and sympathizers in Hollywood, the State Department and elsewhere using as their tool the House Committee on Un-American Activities and the investigative activities of Senator Joe McCarthy (whose subsequently disgraced lawyer was a man named Roy Cohn who, interestingly, years later was the lawyer of a young New York real estate scion named Donald Trump). The intrepid witch hunters of that era used the term 'fellow travelers' for people who weren't actually Communists, but who were sympathizers, or at least acted in tandem with them, or, at a minimum bumped into them in meetings or whatever. If they were merely political sentient and on the left, unaware of the Communist affiliations of some of their political allies, they were 'gullible' fellow travelers. For knowing what was going on, you lost the dismissive but ameliorating modifier 'gullible.'
The concept of 'fellow traveler' made it easier to hold people accountable for things they didn't know they had done. It would be delightful to apply the same standards to Trump and his minions. Let's see, Attorney General Jeff Sessions is a 'gullible' Russian asset. Campaign Manager Paul Manafort, well, maybe not so gullible.
Anyway you cut it, it's pretty un-American.