Mitt Romney is set to win the Illinois Republican presidential primary, beating rival Rick Santorum in the big-state contest.
Exit poll results show Romney was preferred by Illinois primary-goers who say the economy is the top issue in the campaign.
Earlier today, Santorum was unbowed by polls showing Romney with a 15-point lead in Illinois as he rallied conservatives to stay true to their principles rather than give their vote to a moderate just because he is the "choice of the establishment Republicans".
A resounding win in Illinois will provide Romney with momentum ahead of Louisiana's primary on Saturday and contests in Wisconsin, Maryland and Washington, DC on April 3.
Romney 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.
"I voted for Romney because I think he's the best candidate to beat Obama," George Paterson, 77, told AFP after casting his ballot in the affluent Chicago suburb Wilmette.
"I wish he was a little more specific (on policy) but anything's better than what we have now."
Janet Langdon, 36, said she and her husband "love Mitt" and what he did for Massachusetts, where they used to live.
"The country needs to be run more like a business and he can do that," she said.
Investment analyst William Florida usually votes Democrat, but also cast a ballot for Romney in the state's open primary Tuesday and said he would consider voting for the former Massachusetts governor in November.
"I voted for Romney because I think he's more tolerant than the rest of the Republicans," Florida, 46, said.
"I want to see two good candidates this fall rather than one bad one."
While Romney may have a better chance of winning over moderates and independents crucial to winning a general election, he has been weakened by his failure to win over the Republican party's conservative base in the gruelling 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.
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.
The candidates' spouses have also joined the fray, with Santorum's wife, Karen, insisting in a rare television interview that her husband was pro-women and would not seek to block access to birth control, 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 516 delegates, Santorum has 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 won only two contests - to drop out of the race to consolidate the conservative vote.
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.
A Rasmussen national poll gave Obama a six-point lead over Santorum, but a statistical tie with Romney.
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