Victory for Mitt Romney in Illinois Republican primary
Mitt Romney has won the Republican primary in Illinois, his latest win on the road to decide who will contest November's poll against Barack Obama.
Mr Romney has won 47% of the votes, compared with 35% for Rick Santorum, with nearly all votes counted.
Sending a message to President Obama at a victory party in a suburb of Chicago, Mr Romney said: "We've had enough."
Ron Paul polled 9% in Illinois and Newt Gingrich was on 8%; neither candidate campaigned extensively in the state.
"I'm running for president because I have the experience and the vision to get us out of this mess," said Mr Romney, as his victory became evident.
Illinois sends 54 delegates to the Republican convention, and Mr Romney hopes to maintain his momentum.
However, Illinois's delegates are not determined by the statewide vote.
Individual delegates are listed on the ballot in each of the state's 18 congressional districts and are identified by the candidate they support.
Mr Santorum's campaign did not successfully file for the primary ballot in parts of Illinois, meaning he automatically cedes 10 of those delegates.
However, addressing supporters on Tuesday evening, Mr Santorum said he had polled well in Illinois in areas "that conservatives and Republicans populate".
"We're very happy about that and we're happy about the delegates we're going to get, too," he said.
The candidates had clashed in Illinois over the economy.
On Monday Mr Santorum had said he "didn't care about the unemployment rate", and told supporters the presidential campaign was about smaller government and winning back individual and social freedom.
Mr Romney jumped on those remarks later in the day, telling students in Peoria, Illinois "one of the people who is running also for the Republican nomination today said that he doesn't care about the unemployment rate".
"It does bother me. I want to get people back to work," Mr Romney said.
Later Mr Santorum told supporters: "The economy is a big issue. Unemployment is a big issue."
Mr Romney won a convincing victory in Puerto Rico's primary over the weekend, amassing 83% of the votes, but lost to Mr Santorum in recent contests in the South.
A candidate needs to accumulate 1,144 delegates to the August convention in order to secure the nomination.
Analysts say the current figures make that an almost impossible task for Mr Santorum, who has spoken openly in recent weeks about winning enough delegates to stop Mr Romney taking the crown.
Such an outcome would lead to a competitive vote at the Tampa convention, in which Mr Santorum feels he could overcome Mr Romney.
Mr Santorum has vowed to continue, "competing in every state", citing tepid support for Mr Romney even in states the former Massachusetts governor has won.
On Monday, Mr Santorum, a former Pennsylvania senator, said he would "go out and compete in every state".
"I think it's going to be very difficult as this goes on for anybody to get that magic number," Mr Santorum said in an interview with CBS News, adding that chances were increasing of the nomination being decided at the convention.
US media analysis
Reporting for the Washington Post,David Fahrenthold and Philip Rucker saythat while a win in Illinois was essential in order for Mr Romney to extend his lead in the delegate count, Illinois was, in the end, "about more than just numbers".
"After squeaker victories over Santorum in Michigan and Ohio, he needed to show that he could kindle enough voter enthusiasm for a big win outside his north-eastern power base.
"On Tuesday, he got it. For the first time since long-ago Florida, the former Massachusetts governor demonstrated that he could win as big as he spends."
Maggie Haberman, writing for Politico, agreesthat winning the Prairie State was a significant step for Mr Romney, saying it enabled the former governor to look and sound more presidential.
"His win was not a knockout, and Santorum - as well as Newt Gingrich and Ron Paul - will keep going.
"Even so, this was an important pivot point for Romney, who seemed noticeably more at ease in an election-night speech that made no mention of his rivals and that sounded much more like a general election message than the ones he's delivered on past primary nights."
The New Yorker's John Cassidy wrotethat Mr Romney appeared to have found his stride while delivering his victory speech on Tuesday night.
"What nearly four months of trench warfare has really left Romney with is frighteningly high negative ratings from the independent voters he will need to win the White House; a depleted campaign bank account (doubtless he can refill it); and a number of hostages to fortune that the Obama campaign will be eagerly waiting to exploit.
"But last night, perhaps for the first time, the argument that Romney will be a better candidate for what he's been through didn't sound completely outlandish."