Front-runner Mitt Romney sailed to an easy victory in the Illinois primary Tuesday night, trumping Rick Santorum in yet another industrial state showdown and padding his already-formidable delegate lead in the race for the Republican presidential nomination.

"What a night," Romney told cheering supporters in suburban Chicago. Turning his attention past his GOP rivals, he said he had a simple message for President Barack Obama, the Democrat he hopes to face and defeat in November: "Enough. We've had enough."

Returns from 75 per cent of Illinois' precincts showed Romney gaining 47 per cent of the vote compared to 35 per cent for Santorum, nine per cent for Ron Paul and eight per cent for a fading Newt Gingrich.

That was a far more substantial showing for Romney than the grudging victories he eked out in the previous few weeks in Michigan and Ohio, primaries that did as much to raise questions about his ability to attract Republican support as to quell those questions.

Santorum sounded like anything but a defeated contender as he spoke to supporters in Gettysburg, Pa. He said he had outpolled Romney in downstate Illinois and the areas "that conservatives and Republicans populate. We're very happy about that and we're happy about the delegates we're going to get, too."

"Saddle up, like [Ronald] Reagan did in the cowboy movies," he urged his backers.

Santorum tries to overcome self-created controversies

Romney triumphed after benefiting 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 Obama.

Exit poll results showed Romney preferred by primary goers who said the economy was the top issue in the campaign, and overwhelmingly favoured 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 backpedalled 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 Senator Hillary Rodham Clinton to support legislation allowing felons the right to vote.

Initial results showed Romney's victory was worth at least 22 delegates in Illinois.

That gave him 544 in the overall count maintained by The Associated Press, out of 1,144 needed to win the nomination. Santorum has 253 delegates, Gingrich 135 and Paul 50.

Anticipating a primary defeat, Santorum's campaign argued that the race for delegates is closer than it appears.

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

Republican presidential candidate Rick Santorum, accompanied by family members, speaks during a primary night rally on Tuesday in Gettysburg, Pa. (Matt Rourke/Associated Press)

In the long and grinding campaign, Santorum looked to rebound in next Saturday's primary in Louisiana, particularly given Romney's demonstrated difficulties winning in contests across the Deep South.

A 10-day break follows before Washington, D.C., Maryland and Wisconsin hold primaries on April 3.

Santorum is not on the ballot in the U.S. capital.

Private polling shows Romney with an advantage in Maryland, while Wisconsin, an industrial state next door to Illinois, shapes up to be the next big test against Santorum.

Neither Gingrich nor Paul campaigned extensively in Illinois.

Gingrich has faded into near-irrelevance in the race, but he was defiant in a statement issued after Romney sealed his victory.

"To defeat Barack Obama, Republicans can't nominate a candidate who relies on outspending his opponents 7-1. Instead, we need a nominee who offers powerful solutions that hold the president accountable for his failures," it said.

Gingrich said his campaign will spend the time leading to the party convention "relentlessly taking the fight to President Obama."

Both Romney and Santorum campaigned energetically across Illinois, 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."

Illinois primary marks halfway point in Republican campaign

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 Republican 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.