U.S. Republican presidential hopefuls Mitt Romney and Rick Santorum are campaigning in the central state of Illinois ahead of Tuesday's nominating contest.

Former Massachusetts governor Romney, fresh off his victory in the U.S. island territory of Puerto Rico, met with voters at a diner in the capital, Springfield, before heading to Chicago to deliver an economic address. He holds an evening town hall in central Illinois.

While in Springfield, he pledged that he would control government spending.

“I want to restore the principles that made America the greatest nation on Earth — and one of them is you don't spend more than you take in.”

Santorum has said that if he wins Tuesday's primary, he will secure the Republican Party presidential nomination. The former U.S. senator, who is behind Romney in state polls, urged supporters to go out and get others to vote for him.

“This could be an amazing time in American history, and right here in Illinois you can usher that in. You can surprise the pundits, amaze them with conservatives.”

On Sunday, Romney won Puerto Rico's 20 delegates, moving him closer to the 1,144 needed to win the nomination. Romney, who has already won more than 500 delegates, is far ahead of the other candidates in the delegate count.

Meanwhile, the two other Republicans vying for the party nomination, former U.S. House speaker Newt Gingrich and U.S. Representative Ron Paul, were out of the spotlight Monday, with no public events scheduled.

President Barack Obama, a Democrat who will face the eventual Republican nominee in November, campaigned in Chicago, his hometown, last week. He has a re-election event in Washington Monday evening. His campaign said he and his Democratic allies raised $45 million in February for his bid for a second term, with nearly all of the money from contributions of $250 or less.