Black star rising

From back cover Del Ray paperback April 1986:

TAKE US TO YOUR LEADER, IF YOU CAN..." When a mysterious alien spacecraft approaches Earth and demands to speak with the President of the United States, then destroys a large pacific island to demonstrate its strength and underscore its seriousness, you would expect the President to talk. Problem is, in the late twenty-first century, there is no President -- not even a United States. In fact, in this world of the future, China rules the Americas; and, to most people, "USA" and "USSR" are just quaint abbreviations in historical dictionaries. Then the aliens prove unreasonable about accepting substitutes... so one Anglo rice-cultivator from the Heavenly Grain Collective Farm -- near Biloxi, Mississippi -- is forced to begin an adventure that will take him from peasant to President, from Pettyman to Spaceman.