CHICAGO (CBS) — U.S. Sen. John McCain (R-Ariz.) said Monday that the Illinois primary may now be the “defining” contest in the Republican presidential race.

On Monday morning, former Massachusetts Gov. Mitt Romney was preparing to deliver a speech on economic freedom at the University of Chicago, and Rick Santorum was making stops in Rockford, Dixon and other Illinois cities, McCain defeated Romney in the 2008 Republican primary, but is supporting Romney four years later.

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McCain stopped to discuss the contest with Harry Porterfield and Roseanne Tellez on the CBS 2 News at 11 a.m.

“Perhaps the defining primary is now in the heartland of America – in the state of Illinois – a diverse state, and I think that when, and if, Mitt Romney wins tomorrow, it’ll give an inevitability to the campaign that is much needed,” McCain said.

As to whether Romney wins Illinois on Tuesday, McCain said it depends on multiple factors.

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“I think a lot of it is going to depend on voter turnout. I think a lot of it is people really haven’t made up their minds yet,” McCain said. “Frankly, that’s why I’m here in Chicago, in hopes that we can convince those undecided that Mitt Romney is the one they should support.”

McCain added that he believes the Republicans are being damaged by the attack ads and mudslinging that have been going on during the primary in Illinois and elsewhere.

“Every day that goes by, every day as a campaign, you lose a day and you never get it back. And so by concentrating on each other – particularly with these SuperPACs and all these negative ads – then that only benefits President Obama,” McCain said. “And if you’re interested in electing a Republican like Mitt Romney, you want to have this thing come to an end.”

He said Illinois could very well be the deciding factor in the Republican primary.

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“I think if Illinois does big for Mitt Romney, it will be inevitable that he is the nominee,” McCain said.