For me you start with a series of highly disturbing and dangerous comments by Trump about Russia and NATO. As I noted a couple weeks ago, our relative peaceful and prosperous world is far from the natural or only possible order of things. Things can get bleak very quickly, especially when you're stupid. Trump's comments are far more than highly questionable foreign policy ideas. He seems unable to understand high level geopolitics as anything more than a protection racket with him running for American mob boss. Even if Trump is no more than ignorant, impulsive and stupid, these are highly disturbing intimations of a Trump presidency that should have everyone across the political spectrum taking note and re-evaluating what sort of situation we're dealing with.
But that's not all there is. On top of that you have very high level advisors to Trump who have been deeply immersed in the Putin world of dirty politics and energy concessions that characterizes Putin's Russia and the post-Soviet successor states. Those associations might simply be unsavory if the candidate were an experienced political figure or surrounded by knowledgable advisors. Neither is the case.