Front-runner Romney won an easy victory, beating Rick Santorum with 46-percent of the votes to Santorum's 35-percent. Ron Paul followed with 9-percent of the vote and Newt Gingrich had 7-percent. Neither Paul nor Gingrich campaigned extensively in the state.
Romney's victory is worth at least 13 delegates. He's leading with 535 so far, according to The Associated Press. He needs 1,144 to win the Republican nomination. Santorum has 253 delegates, Gingrich 135 and Paul 50.
"What a night," Romney told cheering supporters in suburban Chicago. Turning his attention past his GOP rivals, he said he had a simple message for President Barack Obama, the Democrat he hopes to face and defeat in November: "Enough. We've had enough."
"We know our future is brighter in these troubles times. We still believe in America and we deserve a president who believes in us and I believe in the American people. Each day we move closer not just to victory but to a better America. Join us. Join us. Together we're going to ensure that America's greatest days are still ahead," Romney told his supports Tuesday evening.
Romney spent seven times more than any other candidate on television advertising in the state.
Exit polls showed Republican voters were most interested in the economy and which candidate had the best chance of beating President Barack Obama.
Santorum, who campaigned heavily in downstate Illinois in recent days, conceded Illinois to Romney from Pennsylvania, where he is campaigning for that state's primary.
"I ask each and every one to join us to saddle up like Reagan did in the cowboy movies. To saddle up, to take on that responsibility over the next five weeks. We're heading to Louisiana for the rest of the week and then we're going to be back here in Pennsylvania and we're going to pick up a whole boatload of delegates and close this gap and on to victory," Santorum said.