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.

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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.