There are many reasons to oppose Trump. His understanding of complicated issues and policy is clearly very slight. His courting of Russia is already somewhat of a national embarrassment. His appeals to racist ressentiment. But what really doesn’t get discussed much is just how un-American are the values of both Trump and his campaign.
For example, there is Trump’s dictatorial “only I can solve these problems” rhetoric. This is not republican rhetoric. The essence of republicanism is the idea that what’s important in a country isn’t any one or group of individuals, but rather, a constitution creating limited institutions with robust checks and balances. Individuals of at least reasonable moral and intellectual character are interchangeable within these institutions. Trump’s “great leader” rhetoric is wildly unrepublican, and therefore, wildly un-American (in the sense that to be “American” is to be a little-r republican). Here’s a personal anecdote: my grandfather, now in his mid-90s, originally switched from the Democratic party to the Republican party because he thought FDR was “the closest thing to a dictator in American history.” He told me a few months ago that he was switching back to Democrat for the same reasons (i.e. Trump’s dictatorial and anti-republican rhetoric).