This is my first real attempt to answer a question, to the best of my ability on this sub so forgive me if I manage it mess it up or break a rule.

But it appears there were referenda held in the Northern Mariana Islands when it was a part of the UN Trust Territory of the Pacific Islands (administered by the United States) which included the current Northern Marianas, as well as the Marshall Islands, Palau, and the Federated States of Micronesia. These referenda, held in 1958, 1961, 1963, and 1969, all favored integration with Guam, but Guam rejected unification in a 1969 referendum held in that territory. According to Jonathan H.X Lee in his book Asian American Religion and Cultures, Guamanians rejected unification with the Northern Marianas because they accused the Northern Marianans as being collaborators with Japan in the Second World War, acting as interpreters.

EDIT: I should further add that Lee says that although the Northern Marianas and Guam remain seperate US territories, they still share a very common culture and he says they are unified at least from a cultural perspective today.

I know this probably isn't that detailed but I'm not much of an expert so sorry if it was not informative.