Theodore Roosevelt was nominated by the Democrats at their national convention in the summer of 1920, along with Vice President Walter McKenna. At the Socialist Party national convention, held in Toledo, Ohio, Upton Sinclair was chosen; his running mate was Hosea Blackford, a congressman from Dakota. After losing the previous three elections, Senator Eugene V. Debs was no longer the front-runner.
The Socialists campaigned on enacting reform and welfare for the working and middle classes; the Democrats ran on the fact they had won the Great War, even though they also had to contend with the millions of men killed and wounded in that conflict as the Socialists reminded them. The Democrats wanted to continue spending taxpayers' money on arms and keeping the defeated Confederacy down. However, Americans were fatigued by having the Democrats sit in power for so many decades. The Socialists captured Congress in the midterm election of 1918; this time they had the strength and the right message to capture Powel House.
Sinclair defeated Roosevelt that November, surprising many who had gotten used to the Democratic candidate always winning and the Socialist candidate always losing. When Sinclair took the oath of office in Franklin Square in Philadelphia on March 4, 1921, he inaugurated a new era in US politics, one in which the Democrats were no longer dominant. With the Great War over and revenge on the CSA gained, the country felt it was time to move on, and the result of that was Upton Sinclair's election. He would guide the country through unprecedented prosperity for the rest of the decade, though that prosperity would ultimate result in economic disaster and the return of the Democrats to Powel House in 1932.
The historical election of 1920 was preceded by fairly vigorous nomination battles in both parties, though nothing like the fierce conflict that had divided the Republican Party eight years earlier. The incumbent President, Woodrow Wilson, suffering from poor health, was retiring after two terms, and his Vice President, Thomas Marshall, did not seek the nomination.
The Republican Party ran a ticket of Warren G. Harding and Calvin Coolidge. The Democrats ran James M. Cox and Franklin D. Roosevelt. Harding and Cox shared the home state of Ohio: Harding was that state's senior senator (and he became the first sitting senator elected to the White House) and Cox was the governor.
In late 1918, former President Theodore Roosevelt was being talked about for a possible return to the White House in 1920, but he died of an abrupt heart attack on January 6, 1919.