After his narrow margins of victory 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 was headed to a sizable win over his closest rival, Rick Santorum, in Illinois’s popular vote. His aides were hoping that he would also increase his lead in delegates sufficiently 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 Chicago suburbs, where he was outperforming 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 not only won among the groups that usually support him — moderates, college graduates and wealthier voters — but also was competitive 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 Mr. 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.
As Mr. Romney addressed supporters in a hotel ballroom in the Chicago suburb of Schaumburg, he congratulated his rivals and immediately turned his focus to the president. He belittled Mr. Obama’s experience as a constitutional law professor at the University of Chicago and as a community organizer on the South Side of Chicago, saying the president was ill suited to lead the nation to economic prosperity.
“It’s time to say this word: enough. We’ve had enough,” he said. “We know our future’s brighter than these troubled times. We still believe in America, and we deserve a president who believes in us, and I believe in the American people.”
Using some new lines of attack, he portrayed Mr. Obama as stifling “economic freedom” through an expanding government and overreaching regulators.
Mr. Santorum spoke to supporters in Pennsylvania, his home state, where he will have to win the primary on April 24 to remain a viable alternative to Mr. Romney.
“We won the areas that conservatives and Republicans populate,” Mr. Santorum said of the results in Illinois, where he won large parts of the state outside Chicago. “We’re happy about that. We’re happy about the delegates we’re going to get, too.”
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, a Romney supporter, 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.