(CNN) The big money favorites prevailed on Tuesday in the Illinois gubernatorial primaries, CNN projects, setting the stage for what could be the most expensive governor's race in US history.

Billionaire J.B. Pritzker outlasted and outspent the Democratic field, while Republican Gov. Bruce Rauner, a multimillionaire himself, narrowly beat back an insurgent conservative challenge from state Rep. Jeanne Ives.

Pritzker poured about $70 million into the primary contest, while businessman Chris Kennedy, son of the late Robert F. Kennedy, put up about $2 million of his own cash. State Sen. Daniel Biss, who made a play for progressive Democrats, narrowly leads Kennedy but neither, ultimately, could run with Pritzker, whose campaign overcame a series of high-stakes setbacks while still managing to invest in the groundwork for the coming general election.

"Tonight, we've taken the next step of beating Bruce Rauner and putting Illinois back on the side of working families," Pritzker said in a statement soon after the contest was called.

Rauner's road to victory in the primary, paved in December 2016 with an initial investment of $50 million from his own coffers, turned rocky toward the finish, as Ives, questioning his bona fides on social issues, made a strong push for conservative voters.

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