One ad accused the former Pennsylvania senator of changing his principles while serving in Congress, while two others criticized him for voting to raise the debt limit, raise his own pay as a lawmaker and side with former Sen. Hillary Rodham Clinton to support legislation allowing felons the right to vote.

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In all, Romney and Restore Our Future outspent Santorum and a super PAC that backs him by $3.5 million to $500,000, an advantage of 7-1.

That's about half the percentage in last week's primary states of Alabama and Mississippi, where Santorum won narrowly.

As in other states, the economy was the top-rated issue and an ability to defeat President Barack Obama was the quality that mattered most to primary voters, according to preliminary poll results.

Neither Newt Gingrich nor Ron Paul campaigned extensively in Illinois. Romney and Santorum did, though, and not always in respectful tones.

"Senator Santorum has the same economic lightweight background the president has," Romney said at one point. "We're not going to replace an economic lightweight with another economic lightweight."

Santorum had a tart reply. "If Mitt Romney's an economic heavyweight, we're in trouble."

Including Romney's victory last weekend in Puerto Rico, the former Massachusetts governor had 522 delegates going into the Illinois voting.