Romney a Victor in Illinois Easily, Exit Polls Show Mitt Romney took the stage with his wife, Ann, at his campaign's primary night party in Schaumburg, Ill., after winning the Illinois primary. CHICAGO - Mitt Romney won the Illinois Republican primary on Tuesday, using the full force of his campaign and an argument that he has the best chance of defeating President Obama to overcome doubts among the more conservative voters at the heart of his party. After only narrow victories in battleground states like Michigan and Ohio and losses in Southern states kept alive questions about his ability to rally his party behind him, Mr. Romney seemed to be heading to a sizeable margin of victory in the popular vote in Illinois over his closest rival, Rick Santorum. His aides were hoping his win would be large enough to tamp down talk of a contested convention and build an unassailable advantage in the race for the nomination in the weeks ahead. Illinois is the third state in the industrial Midwest that Mr. Romney has carried in the last month. The electorate, particularly in the suburbs of Chicago, where exit polls showed he was significantly outpacing Mr. Santorum, was also reflective of relatively moderate states that will vote across the Northeast in April. Surveys of Illinois Republicans leaving polling places showed that Mr. Romney won not only among the groups that usually support him - moderates, college graduates and wealthier voters - but also among Mr. Santorum's generally more loyal coalition of Tea Party supporters and married women. If there was one driving force for Mr. Romney, it was the desire of Illinois Republicans to defeat President Obama in the general election. Mr. Romney was supported by nearly three-quarters of those who said in exit polls that winning in November was their top priority. Mr. Romney came here hoping to provide a convincing enough victory to be able to accelerate his move toward directly confronting the president, leaving the intricacies of the delegate fight to his campaign advisers. Yet unless his Republican rivals decide to step aside, Mr. Romney will not be able to move beyond the primary campaign for at least two more months. More important than the popular vote here was the allocation of delegates, who for the first time this year appeared on ballots along with the presidential candidates they are pledged to support in the general election. Mr. Romney was vying to win the most of them, helping his campaign to widen its lead in the competition to reach the 1,144 necessary for the nomination. He far outspent Mr. Santorum here - by a ratio of at least six to one on television - and was favored to win all along. But party leaders expected a Romney victory to "take the steam out of Santorum's argument" for fighting on to the convention, as the Illinois Republican Party chairman, Pat Brady, put it Tuesday. The arcane and often confusing system is forcing the campaigns to navigate a patchwork of state party bylaws that has built up since the era of Abraham Lincoln, who, as it happened, won the Republican nomination in 1860 after swaying delegates to support him in a third round of voting. No ballot better exemplifies the complexities of the system than that of Illinois. When voters here went into the polling booths, they were first asked to choose a presidential candidate, delivering the top-line result that drove the news coverage of the evening. But more important was their selection of up to six delegates in each Congressional district - some of them alternates - who would presumably support the presidential candidate listed next to their names at the convention in Florida in August. But those candidates are "pledged," not bound, meaning they face no penalty for switching sides at the convention. The Illinois Republican Party convention in June will select 15 additional delegates, who would essentially be free agents. In all this year, there are about 500 delegates of that unbound nature, as well as a mix of roughly 1,600 delegates who are either pledged or outright bound to a candidate - or who will be after the formal allocation in a series of county, district and state Republican Party conventions. Those started after March 6 and will continue into July. Projecting the delegate count based on the results so far, or pledges of support, is an inexact science. That explains the vast differences in the scorecards of the campaigns and the news media and clouds the picture of just how likely Mr. Romney is to avoid an open convention by clinching the nomination in or before the last primary, Utah's, on June 26. Going into the Illinois primary, Mr. Romney had 522 delegates, according to The Associated Press, which conducts a survey of party leaders who may serve as delegates and calculates the intentions of unpledged delegates based on the popular vote. But the Republican National Committee says Mr. Romney has 416 delegates, because it does not take unpledged delegates into account. Mr. Santorum had 252 delegates before Illinois, according to The Associated Press, while the Republican National Committee count showed him with 170. Marjorie Connelly contributed reporting from Chicago, and Sarah Wheaton from New York.