SCHAUMBURG, Ill. (AP) -- Backed by a crushing television ad advantage, Mitt Romney jumped ahead of Rick Santorum in early returns from the Illinois primary Tuesday night, bidding for yet another industrial-state triumph in the race for the Republican presidential nomination.
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.
Returns from the first few Illinois precincts showed Romney with 55 percent of the vote, compared to 29 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 President Barack 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, Santorum 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.
While pre-primary polls taken several days ago 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.
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. Santorum had 253, Gingrich 135 and Paul 50. If Romney continues on the same pace, he will lock up the nomination before the convention opens in Tampa, Fla., next August.
However, the Santorum campaign argued Tuesday that the race for delegates is closer than that.
Santorum contends the Republican National Committee at the convention will force Florida and Arizona to allocate their delegates on a proportional basis instead of winner-take-all as the state GOP decided. Romney won both states.
On Tuesday, about four in 10 voters interviewed as they left their polling places said they were evangelical or born again. That's about half the percentage in last week's primary states of Alabama and Mississippi, where Santorum won narrowly. Despite an unusually lengthy race for the nomination, less than a third of those voting said in the polling-place survey they hoped the primary season would come to a quick end even if that meant their candidate might lose the nomination.
The findings came from preliminary results from the survey of 1,078 Illinois Republican voters, and had a margin of error of plus or minus 4 percentage points. The exit poll was conducted for The Associated Press and the television networks by Edison Research at 35 randomly selected polling places around the state.
As Illinois Republicans voted on Tuesday, Romney raised more than $1.3 million at a luncheon in Chicago. He planned an election-night event in nearby Schaumburg, Ill., while Santorum was in Gettysburg, Pa., site of Illinois favorite son Abraham Lincoln's most famous speech.
Santorum, the former Pennsylvania senator, has been seeking to make up in broadcast interviews what he has lacked in advertising money.
On Monday, his campaign began before sun-up and ended well after dark, including four appearances at rallies around the state as well as an extraordinary 19 radio and television interviews. He accused Romney anew of putting his signature on a Massachusetts health insurance law that is similar to the one Obama pushed through Congress.
Romney cut short his planned time in Puerto Rico, site of a primary last weekend, to maximize his time in Illinois. He has eked out victories in other big industrial states over the past few weeks, beginning in Michigan on Feb. 28 and Ohio on March 6. Defeat in any would be likely to trigger fresh anxiety within the party about his ability to wrap up the nomination.
Illinois was the 28th state to hold a primary or caucus in the selection of delegates to the nominating convention, about halfway through the calendar of a Republican campaign that has remained competitive longer than most.
A change in party rules to reduce the number of winner-take-all primaries has accounted for the duration of the race. But so has Romney's difficulty in securing the support of the most conservative of the GOP political base. Santorum and Gingrich have struggled to emerge as the front-runner's sole challenger from the right.
Whatever the reasons, the race appeared unlikely to end soon, with Santorum and even Gingrich vowing to campaign into the convention.
Next up is a primary Saturday in Louisiana where Santorum projects confidence following twin triumphs a week ago in Alabama and Mississippi. There are 25 delegates at stake.
Behind Louisiana is a three-primary night in the District of Columbia, Maryland and Wisconsin on April 3, with 95 delegates combined at stake.
Santorum is not on the ballot in Washington, D.C., but is ahead in opinion polls in Maryland. Wisconsin -- adjacent to Illinois -- shapes up as the most competitive primary of the night.
David Espo reported from Washington