1984 United States presidential election in Illinois

The 1984 United States presidential election in Illinois took place on November 6, 1984. All 50 states and the District of Columbia, were part of the 1984 United States presidential election. State voters chose 24 electors to the Electoral College, which selected the president and vice president of the United States.

Illinois was won by incumbent United States President Ronald Reagan of California, who was running against former Vice President Walter Mondale of Minnesota. Reagan ran for a second time with former C.I.A. Director George H. W. Bush of Texas, and Mondale ran with Representative Geraldine Ferraro of New York, the first major female candidate for the vice presidency.

As of 2020, this is the last time a Republican presidential candidate won over 1 million votes in Cook County, the state's most populous county. The presidential election of 1984 was a very partisan election for Illinois, with over 99% of the electorate voting only either Democratic or Republican, though several other parties did appear on the presidential ballot in the State. Nearly every county in Illinois voted in majority for Reagan. One notable exception to this trend was Chicago's highly populated Cook County, which voted in majority for Mondale, albeit with a 2.6% margin, or 51% to 48.4%. 1984 marks the last time any presidential candidate won Cook County with a single-digit margin, and the last election that a Republican won over a million votes in that county. Fulton, Henderson, Knox, Mercer, and Putnam Counties would not vote Republican again until 2016.

Illinois weighed in for this election as 4 percentage points more Democratic than the national average. As of the 2020 presidential election, this is the last election in which Jackson County voted for a Republican presidential candidate. Reagan won the election in Illinois with a decisive 13 point landslide, carrying all but five counties. No Republican candidate has received as strong of support in the American Great Lakes States, at large, post-Reagan. While Illinois typically voted conservative at the time, the election results in Illinois are also reflective of a nationwide reconsolidation of base for the Republican Party which took place through the 1980s; called by Reagan the "second American Revolution." This was most evident during the 1984 presidential election. Notably, this is the closest to date that a Republican has come to carrying Cook County, home to Chicago, since Richard Nixon won it in 1972. Mondale took 51% of Cook County's vote to Reagan's 48.4%.