Underlying the violent campaign was the lucrative Prohibition-era bootlegging trade, a corrupt city government, politicians with ties to organized crime, and a deep-seated and bitter political rivalry between several of the Illinois Republican candidates. The threat of election day violence was so severe that Chicago's U.S. Marshal requested the U.S. Attorney General for authority to deputize 500 additional federal marshals to assure the electorate to cast their ballots in safety.
The Pineapple Primary took place in 1928, during the administration of the notoriously corrupt Chicago Mayor William Hale ("Big Bill") Thompson, a Republican. Thompson had served two corruption-marred terms as mayor in 1915 and 1919. Following exposure of several scandals tied to his political organization, Thompson sat out the 1923 contest, with the result that reformer William E. Dever, a Democrat, was elected mayor.
After four years away from City Hall, Thompson cast his hat into the ring for the 1927 mayoral campaign, capitalizing on public displeasure with the zealous enforcement of the Prohibition laws under Dever. The always-bombastic Thompson campaigned for a wide open town, at one time hinting that he'd reopen illegal saloons closed by Dever's police. Such a proclamation helped Thompson's campaign gain the support of mobster Al Capone. Thompson's campaign allegedly accepted a contribution of $250,000 ($3522031 in 2017) from the gangster. In the 1927 mayoral race, Thompson beat Dever by a relatively slim margin.
Chicago Mayor "Big Bill" Thompson was the leader of one faction of the state Republican Party. Leading the opposition to the Thompson camp was former Governor and incumbent U.S. Senator Charles Deneen. Thompson and Deneen had been rivals for control of the Illinois Republican Party, and the bad blood between the two politicians dated at least as far back as the 1904 state convention.