White House hopeful Mitt Romney is looking for a big win in President Barack Obama's home state Tuesday as he fights to clinch the Republican nomination and focus on November's general election.
Underdog Rick Santorum was pushing for an upset as he rallied conservatives to stay true to their principles rather than give their votes to a moderate just because he is the "choice of the establishment Republicans."
That upset appears unlikely, however, and a resounding win in Illinois would provide Romney with powerful momentum ahead of Louisiana's primary Saturday and contests in Wisconsin, Maryland and Washington, DC on April 3.
"Mitt Romney is headed for a blowout victory in Illinois," Public Policy Polling said Monday, releasing a survey that showed Romney with a 15-point lead over Santorum, up from a 9-point lead in a Rasmussen poll released Friday.
The former Massachusetts governor also has a commanding lead in the all-important delegate count as he seeks to be the Republican contender to take on Democrat Obama in the November 6 vote.
But Romney has been weakened by his failure to win over the party's conservative base in the grueling state-by-state primary race.
Written off in the early days, Santorum, a former Pennsylvania senator, has steadily notched up wins -- 10 out of the past 31 contests -- largely with the help of evangelicals and the party's most conservative members.
Romney's campaign has spent millions flooding the Illinois airwaves with negative ads calling Santorum an "economic lightweight" and "Washington insider" who has voted against his principles in the past.
Romney set his sights squarely on Obama in a speech at the University of Chicago on Monday in which he cast himself as the defender of conservative economic principles.
"For three years, President Obama has expanded government instead of empowering the American people," he said.
"He's put us deeper in debt. He's slowed the recovery and harmed our economy. And he has attacked the cornerstone of American prosperity: our economic freedom."
Santorum meanwhile wrapped himself in the cloak of late president Ronald Reagan -- a conservative icon -- and insisted Republicans could only beat Obama in November if they elect a "true conservative."
"There's one candidate in this race who can never make this race about freedom because he simply abandoned freedom when he was governor of Massachusetts," Santorum told supporters gathered in Reagan's boyhood home of Dixon, Illinois on Monday.
Santorum also dismissed concerns that his positions are too extreme and could turn off moderate Republicans and key independent voters, noting that the same thing was said about Reagan.
"How can we nominate someone who can't summon the energy, summon the vision, summon the greatness of our country and elevate the debate to something that is big and important and lasting," Santorum said.
The candidates' spouses have also joined the fray, with Santorum's wife Karen defending her husband in a rare television interview with CNN, insisting he was pro-women and that he would not seek to block access to contraception despite his personal opposition to it.
Four Republican contenders are fighting to win the 1,144 delegates needed to lock up the nomination, with the winner to be crowned at the party convention in Tampa in August.
About halfway through the race, Romney has pocketed 516 delegates, while Santorum has won 236 and former house speaker Newt Gingrich has 141, according to the website Real Clear Politics.
Texas Congressman Ron Paul, a small-government champion, has about 66.
Santorum's campaign has been urging Gingrich -- who has only won two contests -- to drop out of the race in order to consolidate the conservative vote.
Illinois and Louisiana between them have more than 110 delegates up for grabs, while by the end of April another 320 or so will have been decided -- many of them in states where the winner takes all.
Should Romney do well in the more liberal East Coast states, as expected, he could well be in an unstoppable position mathematically by the end of next month.
Meanwhile, Obama held a six-point lead over Santorum while he was in a statistical tie with Romney in a national poll released by Rasmussen Monday.