Santorum has all but conceded he cannot earn enough delegates to win, but claimed he was in contest for the long haul because Romney is a weak front-runner.
He said Monday that he'll "go out and compete in every state, calling Illinois a "two-person race."
"What I've said is, I think it's going to be very difficult as this goes on for anybody to get that magic number" to clinch the nomination, Santorum said in an interview on CBS's "This Morning."
He called Romney a "big-government heavyweight," responding on MSNBC Monday to the Massachusetts governor's assertion that he couldn't match up on economic expertise. Santorum told CBS he thinks the chances of a brokered GOP convention in August "are increasing."
In nationally broadcast remarks Sunday, Santorum sidestepped when asked if he would fight Romney on the convention floor if he failed before August to stop the former Massachusetts governor from getting the required number of delegates.
Romney aides privately likened the situation to the Black Knight in "Monty Python and the Holy Grail" who loses his arms and legs in battle with King Arthur but insists he has only a flesh wound. The Romney camp suggested that Tuesday's performance would extend Romney's delegate advantage, even if he loses the popular vote.
Santorum cannot win at least 10 of the state's 54 delegates because his campaign failed to file the paperwork.
One Romney aide recently said it would take "an act of God" for Santorum to earn enough delegates to prevail.
"Mitt's going to do well," said Romney's Illinois chairman Dan Rutherford, the state treasurer. "I think he will do better than the other three people. ... But my focus is on the delegates because that's really what the game is all about."