And I decided for kicks to look before 1972 for the GOP primaries. Most states didn't hold primaries in 1968, but Nixon won New Jersey, Illinois, Wisconsin, and Oregon that year (he lost Ohio, and the rest of the states that have backed every GOP nominee since 1972 didn't have primaries in 1968).
Then going back to 1964, we find that Illinois was the only one of the four above to vote for Goldwater, and thus it wins the title of the longest GOP primary bellwether. It also voted for Nixon in 1960 and Eisenhower in 1956; the last time the winner of the Illinois primary didn't win the GOP nomination was when it voted for Robert Taft in 1952.
In 1964 and 1968, OH had a placeholder favorite son for the eventual nominee. WI arguably had a favorite son in 1964.
