Well, I'm not sure are many of those folks left.
But my own activism on this subject has taught me a couple of things, maybe these will help you:
1 - You will find some, or perhaps, many, who oppose the idea on visceral level, even though they are the some of the loudest critics of "The Swamp". Let's call these folks "American Greatness" people. They have a vision of the US as an international superpower, forever. The lines of the border and the goal of the nation are both fixed, forever, as far as they are concerned. What seems to have some impact is to approach from "you don't have to be huge in size to rule the world: example, England."
2 - Those who fear war and insurrection more than absolute tyranny. Let's call these folks "Peace at all costs" people. Best way to work into their mind is to present it as an ugly divorce. What would most likely lead to violence? A peaceful separation, or forcing two people who loathe each other to stay together until they are at each other's throats? Also, "think of the children!"
3 - Those who fear foreign invasion. Let's call these people The Fortress Americans". Speak to them of who so many other nations are relatively free and yet do not spend trillions on defense. We create artificial power vacuums and crises because of our meddling, peddling and shenanigans in global wars zones. Breaking up would reduce that influence and result in you being less likely to be invaded.
4 - And then there are those who oppose the idea because they oppose you. They will not allow a system where you and your ideas can flourish, because, at the end of the day, they despise you. We'll call these folks Marxists, Jacobins, and Bolsheviks. There is no talking to these people. They want you dead, and do not think for one minute they are going to just let you wander off to do as you please.
