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 and participate in a town hall.

Santorum, who is behind Romney in state polls, has said that if he wins Tuesday's primary, then he will clinch the Republican nomination. The day for the former U.S. senator includes several rallies in Illinois.

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 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.