In 1936, Democratic President Franklin D. Roosevelt defeated Alf Landon in a historic landslide; his party expanded its majority in both houses of Congress.

Young prosecutor Thomas E. Dewey had made a national name for himself as a "Gangbuster" after his convictions of numerous American Mafia figures, including Lucky Luciano and Waxey Gordon.

A Gallup poll following the verdict showed him with a majority of the Republican vote (well ahead of Taft and Senator Arthur Vandenberg) and leading President Roosevelt, 58–42.

These efforts simultaneously alienated New York County party chair Kenneth F. Simpson, an avowed liberal Dewey ally and Hoover critic; instead of supporting Dewey's campaign, Simpson joined with Russell Davenport to support a Democratic businessman for the nomination: Wendell Willkie, president of a utilities holding company and an eloquent critic of the New Deal.

[7] From the start, Dewey engaged in a popular campaign designed to secure the nomination through a show of overwhelming support in the primary elections, whereas Taft and Vandenberg (who avoided making any formal commitment to running) pursued victory through the backroom convention and caucus processes.

The following persons were listed in a major national poll or were the subject of media speculation surrounding their potential candidacy, but declined to actively seek the nomination.

[32] Although internal polling showed him winning easily, the Dewey campaign took an intentionally modest stance, expressing worry over the result and offering to split the delegation with Vandenberg.

[34][35] Mayor of New York Fiorello La Guardia was also submitted as a candidate without his knowledge, but did not sign a statement of candidacy necessary to appear on the ballot and declined to be considered.

With the support of the party's Old Guard, including fellow Senators Arthur Capper, Charles McNary and Gerald Nye, the Vandenberg campaign argued that Dewey was unsound on agricultural issues.

[36] The establishment slate listed as delegates at-large Governor Leverett Saltonstall, Senator Henry Cabot Lodge Jr., U.S. House Minority Leader Joseph W. Martin Jr., and State Treasurer John W. Haigis.

Dewey's name was entered in only three: New Jersey and Maryland, where he was unopposed, and Oregon, where he was soundly beaten as a write-in candidate in favor of Senator Charles McNary.

The events shook the primary race by ensuring the re-enlistment of President Roosevelt as a candidate for re-election to an unprecedented third term and refocusing the election on foreign policy.

Dewey, until then the favorite for the nomination, began to struggle as he attempted to chart an inconsistent path between Vandenberg's isolationism and Roosevelt's aggressive involvement.

[37] On May 10, the night of the invasion of the Netherlands, Ed Jaeckle privately told the Dewey campaign that his chances of being nominated were slim, but he should continue through the convention to ensure control of the New York party and his election as governor in 1942.

[37] The political benefits of the invasion went to Wendell Willkie over the two young favorites; an independent petition drive to nominate the businessman, led by Oren Root, gathered 4.5 million signatures.

As the delegates were arriving in Philadelphia, Gallup reported that Willkie had surged to 29%, Dewey had slipped 5 more points to 47%, and Taft, Vandenberg and former President Herbert Hoover trailed at 8%, 8%, and 6% respectively.

At the 1940 Republican National Convention itself, keynote speaker Harold Stassen, the Governor of Minnesota, announced his support for Willkie and became his official floor manager.

Gallup found the same thing in polling data not reported until after the convention: Willkie had moved ahead among Republican voters by 44% to only 29% for the collapsing Dewey.

The key moments came when the delegations of large states such as Michigan, Pennsylvania, and New York left Dewey and Vandenberg and switched to Willkie, giving him the victory on the sixth ballot.