With most of the precincts reporting, the former Massachusetts governor was well ahead of former U.S. Sen. Rick Santorum of Pennsylvania, U.S. Rep. Ron Paul of Texas and former U.S. House Speaker Newt Gingrich of Georgia. It took just a little more than a half hour after the polls closed for Fox News, CNN, CBS, MSNBC and other media outlets to project Romney the winner.
With 84 percent of precincts reporting, CNN said Romney was ahead with 47 percent of the Republican vote to 35 percent for Santorum, 9 percent for Paul and 8 percent for Gingrich.
The win gives Romney -- who made a last pass at voters as they headed for the polls in the state's first significant GOP presidential primary in decades -- added momentum in his quest for the GOP nomination.
At stake were 54 delegates to the Republican National Convention. Also at stake in Illinois are congressional seats pitting incumbents against each other in the first test since districts were redrawn to reflect the 2010 census.
In a victory speech, Romney told supporters in Chicago the November election will offer voters a stark choice.
"Our choice will not be about party or even about personality," he said. "This election will be about principle. Our economic freedom will be on the ballot."
Afterward, Santorum, addressing supporters in Gettysburg, Pa., said Romney was following his lead on making the election about freedom. He said his campaign is "feeling very, very good about winning Louisiana on Saturday," and he promised an all-out campaign in Pennsylvania, where he said he expected to "pick up a whole boatload of delegates" to the Republican convention in August in Tampa, Fla.
"We've got five weeks, five weeks to a big win and five weeks to a delegate sweep in Pennsylvania," said Santorum, who had not gotten his name on the ballot in all of Illinois' congressional districts.
Romney attended fundraisers and toured Google's Chicago offices before heading to suburban Schaumburg to watch the returns, the Chicago Tribune reported.
Speaking a day ahead of the primary, Romney had argued to his audience of students in Chicago it is free markets and free people, not government regulation, that create economic prosperity.
"The Obama administration's assault on our economic freedom is the principal reason why the recovery has been so tepid -- and why it couldn't meet their expectations, let alone ours," Romney said, wearing a suit and tie and reading from two TelePrompTer screens -- a practice for which Republicans have sharply criticized Obama.
Hours later, Santorum told an audience in Moline, Ill., the economic argument was not central to his White House race.
"I don't care what the unemployment rate's going to be," he said. "It doesn't matter. My campaign doesn't hinge on unemployment rates and growth rates. There's something more foundational that's going on here."
Gingrich, campaigning in Louisiana ahead of that state's Saturday primary, told a news conference in Shreveport Obama was wrong in his assessment of oil and gas prices.
"I think that it is very important to recognize that the president's arguments on gas and energy in fact don't make sense," he said in a city that has largely transitioned to a service economy but still has a strong oil background.
Paul's campaign said the congressman was looking past Illinois to a Friday "Give Me Liberty" grassroots fundraising "money bomb."
Among Illinois' congressional races, Rep. Jesse Jackson Jr. soundly defeated Debbie Halvorson of Crete, a former one-term U.S. representative, in the Democratic primary, the Chicago Sun-Times reported. With 86 percent of the district's precincts reporting, Jackson had 71 percent of the vote, the newspaper said.