Illinois held a non-binding primary. Kennedy ran a write-in campaign, and no candidate actively ran against him in Illinois.
Rather than being determined by the primary, the selection of delegates was determined by local Democratic organizations. Chicago mayor Richard J. Daley, head of the Cook County Democratic Party, promised to deliver Kennedy of Cook County's delegates, so long as Kennedy won competitive primaries in other states.
Some, including Republican legislators and journalists, believed that Kennedy benefited from vote fraud from Mayor Richard Daley's powerful Chicago political machine. Republicans tried and failed to overturn the results at the time—as well as in ten other states. Some journalists also later claimed that mobster Sam Giancana and his Chicago crime syndicate "played a role" in Kennedy's victory. Nixon's campaign staff urged him to pursue recounts and challenge the validity of Kennedy's victory, however, Nixon gave a speech three days after the election stating that he would not contest the election.
A myth arose that Chicago Mayor Richard J. Daley held back much of the city's vote until the late morning hours of November 9. Yet when the Republican Chicago Tribune went to press, 79 percent of Cook County precincts had reported, compared with just 62 percent of Illinois's precincts overall. Moreover, Nixon never led in Illinois, and Kennedy's lead merely shrank as election night went on.Earl Mazo, a reporter for the pro-Nixon New York Herald Tribune and his biographer, investigated the voting in Chicago and "claimed to have discovered sufficient evidence of vote fraud to prove that the state was stolen for Kennedy."
However, a special prosecutor assigned to the case brought charges against 650 people, which did not result in convictions. Three Chicago election workers were convicted of voter fraud in 1962 and served short terms in jail. Mazo, the Herald-Tribune reporter, later said that he "found names of the dead who had voted in Chicago, along with 56 people from one house." He found cases of Republican voter fraud in southern Illinois, but said that the totals "did not match the Chicago fraud he found."