The endorsement was the first that the Cook County Republican Party has made in a gubernatorial primary in at least 25 years. Cook County Republicans represent about 20 percent of GOP primary voters across Illinois.
Cook County GOP officials have said they want to do a better job of backing the strongest candidate in the governor's race. They have said making an endorsement is a show of cohesion that could boost voter turnout for the group's candidate of choice in the state's most populous county.
"For the first time in recent memory, Republicans in the largest county in Illinois have a strong, public voice in the Republican primary process," Cook County GOP chairman Aaron Del Mar said.
Rauner, the only candidate from the county, earned 63.3 percent support. State Sen. Kirk Dillard of Hinsdale received 10.1 percent. State Treasurer Dan Rutherford of Chenoa earned about 6 percent and state Sen. Bill Brady of Bloomington received no votes.
Dillard, who has won the endorsement of Republicans in DuPage, a traditional GOP stronghold, has suggested that a Cook County endorsement could work against a candidate anxious to show he is independent of the Chicago area's predominance in the state Capitol.
Across the state, 765,534 Republicans voted in the 2010 primary, the last time there was a race for governor, according to data from county clerks. About 162,000 voters turned out in Cook County in the 2012 GOP primary.