He approached Herbert Hoover about running for the 1920 Democratic presidential nomination, with Roosevelt as his running mate.[88]

Campaign for vice president (1920)
Cox and Roosevelt in Ohio, 1920
Roosevelt's plan for Hoover to run for the nomination fell through after Hoover publicly declared himself to be a Republican, but Roosevelt decided to seek the 1920 vice presidential nomination. After Governor James M. Cox of Ohio won the party's presidential nomination at the 1920 Democratic National Convention, he chose Roosevelt as his running mate, and the convention nominated him by acclamation.[89] Although his nomination surprised most people, he balanced the ticket as a moderate, a Wilsonian, and a prohibitionist with a famous name.[90][91] Roosevelt, then 38, resigned as Assistant Secretary after the Democratic convention and campaigned across the nation for the party ticket.[92]
During the campaign, Cox and Roosevelt defended the Wilson administration and the League of Nations, both of which were unpopular in 1920.[93] Roosevelt personally supported U.S. membership in the League of Nations, but, unlike Wilson, he favored compromising with Senator Henry Cabot Lodge and other "Reservationists".