Other than the links posted by iiandyiii, I’d take into considering how conservatives have treated groups they didn’t like historically. Blacks, muslims or gays. Conservatives have a long, rich history of trying to take away their rights.
Also the president is a clear and present danger to both national security and the constitution. Almost every day he does something that threatens the constitution, the rule of law, balance of powers, etc. and his base could care less. The deafening silence from Trump’s voters as Trump tries to use the justice department to persecute his enemies, tries to engage in large scale voter suppression, tries to obstruct justice, collaborates with hostile foreign nations, violates the emolument clause, hostility to a free press, hostility to freedom of religion, hostility to freedom of assembly (NFL protests for example), encouragement of torture, wanting rapid justice before a trial (which he has done before in the 80s before the recent NYC terror attack), hostility to an independent judiciary, wanting to gather up data on what his critics do online, etc speaks volumes. Trump is a living, breathing threat to democracy, the constitution and the rule of law and 63 million people voted for him and the vast majority don’t care about the risk he poses.
For many of us it is virtually impossible to rectify being a Trump supporter with being someone who cares about the constitution, rule of law, western values, decorum and democracy. Trump is not just ‘another republican’, there is something deeply wrong with him legally, morally and mentally.
So yes, calling Trump and a big % of his voters a threat to the US is a valid statement. If I give a gun to a known criminal and shrug my shoulders when he goes on a killing spree, I’m guilty too even if I don’t explicitly say I support him pulling the trigger every time he did it, and only supported some of the times he did it. Same with Trump and his voters. You can’t claim Trump’s voters care about the constitution or rule of law when they elect someone who is a major threat to these things.