SCHAUMBURG, Ill. (AP) - Front-runner Mitt Romney won the Illinois primary with ease Tuesday night, defeating Rick Santorum in yet another industrial state showdown and padding his already-formidable delegate lead in the race for the Republican presidential nomination.
Romney triumphed after benefitting from a crushing advantage in the television advertising wars, and as his chief rival struggled to overcome self-imposed political wounds in the marathon race to pick an opponent to Democratic President Barack Obama.
Returns from 29 percent of the state's precincts showed Romney gaining 55 percent of the vote compared to 28 percent for Santorum, 9 percent for Ron Paul and 7 percent for Newt Gingrich.
Preliminary exit poll results showed Romney preferred by primary goers who said the economy was the top issue in the campaign, and overwhelmingly favored by those who said an ability to defeat Obama was the quality they most wanted in a nominee.
The primary capped a week in which the two campaigns seemed to be moving in opposition directions - Romney increasingly focused on the general election battle against Obama while Santorum struggled to escape self-created controversies.
Most recently, he backpedaled after saying on Monday that the economy wasn't the main issue of the campaign. "Occasionally you say some things where you wish you had a do-over," he said later.
Over the weekend, he was humbled in the Puerto Rico primary after saying that to qualify for statehood the island commonwealth should adopt English as an official language.
While pre-primary polls taken several days ago in Illinois suggested a close race, Romney and Restore Our future, a super Pac that backs him, unleashed a barrage of campaign ads to erode Santorum's standing.
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.
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.