Warren G. Harding was elected president in 1920, which was a victory for isolationists in the Republican party for many reasons.
First, he defeated the Republican governor of Massachusetts Calvin Coolidge for the party nomination, and Coolidge was more in favor of intervention than Harding, but not by much. Coolidge was wary of alliances, but he would send the U.S. military aboard on several occasions during his eventual presidency.
He also defeated Democrat James M. Cox for the presidency, who favored American membership in the League of Nations as well as trying to push ratification of the treaty through congress.
By defeating both these more interventionist opponents, the Republicans were able to enjoy Harding and his “return to normalcy” in which he pledged to return the U.S. to isolationism and inward focused policies.