Mitt Romney added to his lead among candidates for the Republican U.S. presidential nomination, winning Sunday's party primary in Puerto Rico by trouncing rival Rick Santorum and scoring all 20 of the U.S. Caribbean territory's delegates.

Romney so far has collected more delegates than his opponents combined and is poised to win the delegate battle on Tuesday in Illinois, even if he loses the popular vote, partly due to missteps by Santorum's campaign, the Associated Press reported.

Romney's confidence has been seen in his intensifying calls - public and private -- for his Republican rivals to concede defeat in the nomination battle. The front-runner's wife, Ann Romney, reinforced that sentiment Sunday night in suburban Illinois.

We need to send a message that it's time to coalesce, she said, her husband at her side. It's time to get behind one candidate and get the job done so we can move on to the next challenge, bringing us one step closer to defeating Barack Obama.

Romney aides have steadfastly emphasized their campaign's overwhelming mathematical advantage in the race to 1,144 delegates -- the number needed to clinch the Republican nomination and face President Barack Obama in the November election.

Santorum has all but conceded he can't earn enough delegates to win, but the former U.S. senator from Pennsylvania claimed he was in the race for the long haul because Romney is a weak front-runner.

This is a primary process where somebody had a huge advantage, huge money advantage, huge advantage of establishment support and he hasn't been able to close the deal and even come close to closing the deal, Santorum said Sunday. That tells you that there's a real flaw there.

But Santorum evaded the question of whether he would fight Romney on the floor of this summer's Republican National Convention if he failed before August to stop the former Massachusetts governor from getting the required number of delegates.

Santorum can't win at least 10 of the 54 delegates at stake Tuesday in Illinois because his campaign failed to file required paperwork in the state. The maximum Santorum could gain Tuesday is 44 delegates.

One Romney aide recently said it would take an act of God for Santorum to earn enough delegates to prevail, the AP reported.

On Sunday, the candidates engaged in frenetic, late-stage campaigning across Illinois, the Chicago Tribune reported.

Santorum, who returned to the state after spending the early part of the day campaigning in Louisiana, impugned the oft-touted management skills of Romney, a successful businessman and former Massachusetts governor.

The real question you should ask ... is Governor Romney, why, with tens and hundreds of millions of dollars, hasn't he been able to do anything to get this nomination even close to cemented away? Santorum said on CNN's State of the Union. That shows a real weakness in his ability to be able to govern.

Tuesday's primary in Illinois once again finds Romney campaigning heavily in a state that had once been thought of as simply another cog toward his inevitable march to the Republican nomination.

Instead, concerns over lackluster voter turnout and intensity of support for Romney in suburban areas has made the state a question mark as Santorum has worked to energize Tea Party and religious conservatives, especially outside Chicago.

In his appearances Sunday, Romney reset his focus on the economy by seeking to stoke voter anger toward rising gasoline prices. He blamed Obama, saying the president has opposed drilling in the Gulf of Mexico and blocked the Keystone XL pipeline from Canada.

As Romney traveled, his campaign highlighted Santorum's backing of then-Sen. Arlen Specter of Pennsylvania for president in 1996. With Santorum on stage, Specter announced his bid by saying the Republican Party had strayed too far right, particularly in opposing abortion rights. Asked about the endorsement on ABC's This Week, Santorum said: Look, you know, you work together as a team for the state of Pennsylvania, and I felt that Senator Specter stood up and supported me when I was running in 1994, and I did likewise. I certainly knew Arlen Specter was going nowhere. I certainly disagreed with a lot of things that he said, and it was something I look back on and wish I hadn't done.

Santorum also responded to Romney's accusation that the former senator is an economic lightweight.

If Mitt Romney's an economic heavyweight, we're in trouble, Santorum said, citing Romney's tenure as Massachusetts governor.