Romney's comfortable win gave him a fresh burst of momentum in the White House campaign and new ammunition for his argument that it is time for Republicans to rally around his campaign and end an increasingly bitter nominating battle.
Elections are about choices, and today hundreds of thousands of people in Illinois have joined millions across the country to join our cause, Romney told cheering supporters in Schaumburg, Illinois, a Chicago suburb.
I'm offering a real choice and a new beginning, he said, touting his business experience and ignoring his rivals to focus his criticism on President Barack Obama. I have the vision and the experience to get us out of this mess.
With 62 percent of the votes counted, Romney had 47 percent and Santorum, a former U.S. senator from Pennsylvania, had 35 percent. Romney's remaining rivals, Newt Gingrich, the former U.S. House of Representatives speaker, and libertarian congressman Ron Paul, lagged badly. Gingrich was in fourth place.
Though he has failed to convince many conservatives, Romney has more than twice as many delegates to the nominating convention as Santorum, and Romney's campaign has argued that his rivals cannot catch him in the contest to pick a challenger to Obama in the November 6 election.
Santorum and Gingrich hope to keep Romney from capturing a majority of delegates by the time the nominating contests end in June, leaving the choice up for grabs among the party's mostly conservative delegates heading into the August nominating convention in Tampa, Florida.
We don't need a manager, Santorum told supporters in Gettysburg, Pennsylvania, after the results were announced. We need someone who is going to pull government up by the roots and throw it out.
The win in Illinois allowed Romney to expand his lead in delegates over Santorum. Heading into the voting, Romney had 521 delegates to Santorum's 239 in the race for the 1,144 needed to win the nomination, according to a count by CNN.
There were 54 delegates to be allocated in Illinois from Tuesday's voting, but Santorum was not eligible to win many of them because he failed to meet deadlines to file a slate of delegates in some districts.
Romney and Santorum had launched a new round of attacks on each other ahead of the vote, with Romney calling his rival an economic lightweight and Santorum accusing Romney, the former head of a private equity firm, of being a Wall Street financier who would have a hard time winning in November.
Santorum, a staunch social conservative, campaigned heavily in rural southern Illinois and courted conservatives who distrust Romney for the moderate stances he took as governor of liberal Massachusetts.
But the Illinois result followed a bad week for Santorum, who spent two days campaigning in Puerto Rico before losing badly there on Sunday. In an effort to minimize the importance of Romney's business experience, he also told a campaign rally he did not care about the unemployment rate.
The diverse electorate in Illinois paid off for Romney, who has been unable to translate his substantial financial and organizational advantages into broad support in many states.
Exit polls showed Santorum won among voters who said they were Evangelical Christians, very conservative or strongly supported the conservative Tea Party movement, but those blocs were smaller than in states where Santorum has done well. Evangelicals made up about four of every 10 voters in Illinois, compared to more than seven in 10 last week in the Mississippi and Alabama primaries, which Santorum won.
Romney, a multimillionaire who has struggled to connect with blue-collar voters, slightly led Santorum as the candidate who best understood the average Americans' problems, exit polls showed.
Despite his poor finish, Gingrich pledged to keep pushing until the convention.
This campaign will spend between now and when the delegates vote in Tampa relentlessly taking the fight to President Obama, Gingrich said in a statement.