With 97% votes counted in Tuesday's primary, the former Massachusetts governor Romney had nearly 47% votes to his top rival former senator Rick Santorum's 35%.
After close encounters in battleground states like Michigan and Ohio and losses in southern states, frontrunner Mitt Romney scored a decisive win in Illinois Republican nomination race to challenge President Barack Obama in November.
With 97% votes counted in Tuesday's primary, the former Massachusetts governor Romney had nearly 47% votes to his top rival former senator Rick Santorum's 35% in Obama's home state.
House member Ron Paul was third with over nine%, while former House speaker Newt Gingrich finished last with less than 8%.
"We thank the people of Illinois for this extraordinary victory," Romney told supporters in the Chicago suburb of Schaumburg. "Elections are about choices. Today, hundreds of thousands of people in Illinois joined millions of people in this country in this cause."
The New York Times attributed Romney's big victory to his use of "full force of his campaign and an argument that he has the best chance of defeating President Obama to overcome doubts among the more conservative voters at the heart of his party."
Santorum led among blue-collar voters, in rural areas and among those who considered themselves "very conservative," according to the exit polls.
But the surveys showed Romney leading heavily among the more than 50% of voters who ranked the economy as the No. 1 issue in the campaign.
And among the 37% who considered the chances of beating Obama in November their top quality in a candidate, three-quarters said they were voting for Romney.
Santorum spokesman Hogan Gidley said his candidate plans to continue campaigning in upcoming contests in Louisiana, Wisconsin and North Carolina.
The 54 delegates at stake Tuesday night will be awarded proportionately, and they are likely to further pad Romney's estimated lead, putting him closer to the 1,144 needed to clinch the GOP nomination.
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