Polls in Illinois have closed after a day in which Republicans cast their ballots to select their candidate to take on President Barack Obama, a Democrat, in November.

Heading into the primary, Republican frontrunner Mitt Romney appeared positioned to widen his lead over top rival Rick Santorum.

Opinion polls released a day ahead of the vote showed Romney as many as 15 points ahead of Santorum.

Fifty-four delegates are at stake in the state, and a win would give Romney major momentum. The former Massachusetts governor has already won more than 500 delegates, while Santorum is in the 200s. The other two Republican candidates, Newt Gingrich and Ron Paul, are far behind.

Both Romney and Santorum campaigned in Illinois on Monday. 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.