Judging by the county results and the issues of each election, I'd imagine 1988.
It depends upon your definition of "urban," but probably 1972!
Nixon received a significantly higher percentage of the vote that year than Eisenhower did in 1956, and the urban non-white vote was considerably smaller in 1956 than in 1972.
It was also considerably more Republican. Then again, the urban Black vote was considerably more Republican in 1956.
Still: Depending on definition, any of the dates mentioned, 1928, and 1924 are possible contenders.
