There you have the foreign policy reason to oppose him.
He appeals to the ancient US myth that it can isolate itself from the rest of the world and the more modern one that it is the only country bearing the burden of war.
This is in many ways a world war. The autocracies are collectively attacking the West.The thing in Ukraine is really the same thing as Iran attacking Israel by proxy and China threatening Taiwan. If we want the mostly peaceful and orderly world we have, then we all have to fight against the chaos the autocrats are threatening us with.
And the buffoonish but also evil and demented GOP candidate is on the other side.