WASHINGTON (AP) -- Mitt Romney says he's "almost there" in his six-year pursuit of the Republican presidential nomination after defeating Rick Santorum in the Illinois GOP primary.
"Tonight's win means we are that much closer to securing the nomination, uniting our party, and taking on President Obama," the Republican front-runner wrote in a campaign email sent late Tuesday.
He urged the party to fall in line behind his bid, saying, "We are almost there."
The former Massachusetts governor and his allies spent hundreds of thousands of dollars more than Santorum and his backers in Illinois, and it showed in the results: Romney was beating Santorum by 47 percent to 35 percent.
Romney was the clear favorite among Illinois Republicans who were most concerned about picking someone who is capable of taking on President Barack Obama in the fall. Romney's wife, Ann, suggested earlier this week that it was time for the party to coalesce behind him. And in an appeal to the centrist independents who will decide the general election, Romney pledged Tuesday to work with Democrats or "die trying."
"Tonight was a primary, but November is a general election. And we're going to face a defining decision as a people," Romney said during a victory speech to supporters. "We know what Barack Obama's vision is. We've been living it these last three years. My vision is very, very different."
Romney picked up at least 41 delegates in Illinois, according to initial results, adding to his delegate lead and making it that much harder for any of his rivals to catch him.
Still, Romney could face defeat in some Southern states.
Santorum, who hopes to rebound Saturday in the Louisiana primary, sounded like anything but a defeated contender Tuesday night as he spoke to supporters in Gettysburg, Pa. He said he had outpolled Romney in downstate Illinois and the 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 before invoking Illinois-born Republican icon Ronald Reagan, the actor turned president. "Saddle up, like Reagan did in the cowboy movies."
Former House Speaker Newt Gingrich didn't speak to supporters Tuesday, instead putting out a written statement. Texas Rep. Ron Paul has yet to win a state.
Romney triumphed in Illinois after benefitting from a crushing 7-1 advantage in the television advertising wars, and as his chief rival struggled to overcome self-imposed political wounds in the marathon race to pick an opponent to Obama.
Most recently, Santorum backpedaled after saying Monday that the economy wasn't the main issue of the campaign. "Occasionally you say some things where you wish you had a do-over," he said later.
Romney has 563 delegates in the overall count maintained by The Associated Press, out of 1,144 needed to win the nomination. Santorum has 263 delegates, Gingrich 135 and Paul 50.
After the Louisiana primary, a 10-day break follows before Washington, D.C., Maryland and Wisconsin hold primaries on April 3.
Santorum is not on the ballot in the nation's capital. Private polling shows Romney with an advantage in Maryland, and the pro-Romney super PAC Restore Our Future launched a television ad campaign in the state during the day at a cost of more than $450,000.
Wisconsin shapes up as the next big test between Romney and Santorum. Republican politics there have been roiled recently by a controversy involving a recall battle against the governor and some GOP state senators who supported legislation that was bitterly opposed by labor unions.
Already, Restore Our Future has put down more than $2 million in television advertising across Wisconsin. Santorum has spent about $50,000 to answer.
Neither Gingrich nor Paul campaigned extensively in Illinois. Gingrich has faded into near-irrelevance in the race, but he remains defiant.
"To defeat Barack Obama, Republicans can't nominate a candidate who relies on outspending his opponents 7-1," he said in a statement Tuesday night. "Instead, we need a nominee who offers powerful solutions that hold the president accountable for his failures."
Gingrich said his campaign will spend the time leading to the party convention "relentlessly taking the fight to President Obama."