Illinois has effectively turned into a two-man race, with neither of the other two challengers, Newt Gingrich or Ron Paul, making much effort in the state.
Santorum, in a television interview on Monday morning, made a series of pointed remarks about Romney that almost certainly rule him out of consideration as Romney's vice-presidential running-mate.
Speaking on CBS, Santorum claimed Romney "is someone who doesn't have a core. He has been on both sides of almost every single issue in the past 10 years". That is a damning accusation for a politician, one that the Democrats could happily use as an ad in the fall.
At a rally on Sunday, Santorum, reacting to Romney's jibe that he is an economic lightweight, offered the Democrats another gift, suggesting that American voters were unlikely to want to see a Wall Street financier as president.
"I heard governor Romney here called me an economic lightweight because I wasn't a Wall Street financier like he was. Do you really believe this country wants to elect a Wall Street financier as the president of the United States? Do you think that's the experience that we need? Someone who's going to take and look after as he did his friends on Wall Street and bail them out at the expense of Main Street America," said Santorum.
He added that Romney's boast that he had a good record of job creation while governor of Massachusetts was untrue: it had been the third worst of the 50 states.
A poll by Public Policy Polling Monday showed Romney opening up a wide lead over Santorum after weekend polls had showed Romney with only a slight lead. The poll put Romney on 45%, Santorum 30%, Gingrich 12% and Paul 10%.
Santorum was campaigning throughout Illinois Monday but he may have made a strategic mistake in not concentrating on the state. He spent two days campaigning in Puerto Rico at the end of last week, and in Louisiana Sunday, whereas Romney has campaigned intensively in Illinois Saturday, Sunday and Monday.
Romney's received a minor boost on Sunday when he won Puerto Rico by 85%. With the Puerto Rico results included, Romney has now 521 delegates to the party convention in August where the Republican candidate to face Obama will be chosen. Romney took all 20 of the Puerto Rico delegates.
Santorum has 253 delegates, Gingrich has 136 and Paul 50, according to the AP.
But, with more than half the states having voted, Romney remains well short of the 1,144 delegates he needs to secure a majority of the delegates. Santorum predicts that Romney will fall short and that the issue will have to be fought out at the convention.
He told CBS: "I think it's going to be very difficult as this goes on for anybody to get to that magic number. And what I'm going to do is continue to work hard to make sure there's a conservative who's the nominee of this party … We have proven in the past when we nominate moderates, when we nominate, you know, a Tweedledum versus Tweedledee, we don't win elections."
Romney is hoping to do well Tuesday in Chicago and the relatively well-off suburbs that surround it while Santorum has been concentrating on cities and rural areas to the south. On the campaign trail in recent days, the Romney campaign has targeted women upset over Santorum's views on contraception and abortion while Santorum on Sunday targeted Christian evangelicals, his core supporters.
Intellpuke: You can read this article by Guardian Washington, D.C., Bureau Chief Ewen Macaskill, reporting from Chicago, Illinois, in context here: www.guardian.co.uk/world/2012/mar/19/scrambling-santorum-lashes-out-at-romney