SCHAUMBURG, Ill. (AP) - Backed by a crushing television ad advantage, Mitt Romney sought a strong Illinois primary victory Tuesday to solidify his lead over Rick Santorum in the battle for the Republican presidential nomination.

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It was the latest-in a string of must-win industrial state contests for the front-runner. Romney held a second advantage as well, this one in the competition for Illinois delegates to the party convention next summer.

Santorum was ineligible for 10 of the 54 at stake after failing to field a full slate.

About four in 10 voters interviewed as they left their polling places said they were evangelical or born again.

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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, according to The Associated Press count.