The Cook County Democratic Party is an American county-level political party organization which represents voters in 50 wards in the city of Chicago and 30 suburban townships of Cook County. The organization has dominated Chicago politics since the 1930s. It relies on an organizational structure of a ward or township committeeperson to elect candidates. At the height of its influence under Richard J. Daley in the 1960s when political patronage in employment was endemic in American cities, it was one of the most powerful political machines in American history. By the beginning of the 21st century the party had largely ceased to function as a machine due to the legal dismantling of the patronage system under the Shakman Decrees issued by the federal court in Chicago. The current Chair is Toni Preckwinkle, who is also the elected Cook County Board president.

Cook County is the most populous county in the U.S. state of Illinois and the second-most-populous county in the United States, after Los Angeles County, California. More than 40 percent of all residents of Illinois live within Cook County. As of 2020, the population was 5,275,541. The county seat is Chicago, the most populous city in Illinois and the third most populous city in the United States. The county is at the center of the Chicago metropolitan area.