"Some of these states who typically are force-fed an establishment nominee at this point are getting to see the campaign up close and personal," Gidley said. "They're going to get to see Speaker Gingrich. They're going to get to see Mitt Romney. They're going to get to see Rick Santorum. We'll see, as we move into the convention, just who people are gravitating behind."
"We do acknowledge that it's difficult for any candidate to get to a majority prior to the national convention," Santorum delegate director John Yob said. "If this race goes to the floor of the national convention, we view that at as favorable terrain for Santorum because the delegates to the national convention will by and large be more conservative than the primary voters who voted in the binding contests."
Gingrich and Paul plan to head into Louisiana, which holds its GOP primary Saturday. But CNN analyst Erick Erickson, a longtime Romney critic, said "the writing's on the wall" for the rest of the field.
"This comes down to Mitt Romney," Erickson said. "Not only is he the front-runner, but the nominee. This is a clear win for Mitt Romney tonight in a state with blue-collar voters, with industrial voters and suburban voters."
Romney's win further pads his estimated lead. But for the former Massachusetts governor, who has struggled to win over the conservative GOP base, a clear popular-vote win might be just as important.
He finished third in Alabama and Mississippi primaries a week ago, behind Santorum and Gingrich. But unlike the Deep South states, Illinois has a large, somewhat moderate GOP electorate concentrated around Chicago.
Romney regained a bit of momentum on Sunday when he trounced Santorum in Puerto Rico's primary. He got 83% of the vote, picking up all 20 delegates at stake, and cutting back on time in Puerto Rico to make weekend stops in Illinois. He also dropped his emphasis on the math that he argued made it clear that he alone has the only shot during the primary and caucus season to clinch the nomination.
Over the past few days, he appeared to be increasing his attention to female voters and to gas prices, as well as touting his business credentials and suggesting Santorum would be an "economic lightweight." In response, Santorum asked if Americans wanted a president with ties to big banks.
"I heard Gov. Romney here called me an 'economic lightweight' because I wasn't a Wall Street financier like he was. Do you really believe this country wants to elect a Wall Street financier as the president of the United States?" Santorum asked at a campaign rally in Rockford.