U.S. Republican presidential candidate Mitt Romney is hoping for a big win in his party's primary Tuesday in the central state of Illinois, as he seeks to enlarge his lead over his top rival, Rick Santorum.

Opinion polls show Romney is 15 points ahead of Santorum in Illinois.

Sixty-nine delegates are at stake. A win would give Romney major momentum heading into upcoming nominating contests. The former Massachusetts governor has already won 521 delegates to 253 for Santorum.

The other two Republican candidates, Newt Gingrich and Ron Paul, are far behind.

On Monday, Romney and Santorum campaigned in Illinois. Romney met with voters at a diner in the capital, Springfield, before heading to Chicago to deliver an economic address. He pledged to 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 presidential nomination. The former U.S. senator urged supporters to 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 Republican nomination. The winner will face

President Barack Obama, a Democrat, in the November election.