The race for the Republican nomination is effectively over if Donald Trump wins Wisconsin, Karl Rove said Tuesday.

"If he wins Wisconsin, the contest is over," the former George W. Bush White House aide and Fox News contributor said. "If not, it's gonna go on and the math becomes somewhat more difficult."

Trump leads the GOP field but is still 500 delegates shy of clinching the nomination. Ted Cruz trails Trump by more than 260 delegates but holds a 5-point advantage heading into Wisconsin's primary, according to a RealClearPolitics average of state polls. Rove, however, suggested Wisconsin's open primary process favors the New York billionaire.