Mitt Romney won the GOP presidential primary in Illinois on Tuesday, walloping rival Rick Santorum in a key state whose voters are a bellwether for Republicans nationwide.

With close to 99 percent of precincts reporting, Romney was leading Santorum by almost 12 percentage points, and victory would allow him to extend his already-imposing lead in the race for delegates.

For Romney, however, this was 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 Northeastern 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.

“Today, hundreds of thousands of people in Illinois have joined millions of people around the country to join our cause,” a confident Romney told supporters in Schaumburg, Ill., on Tuesday night. He said nothing about his Republican rivals but attacked President Obama as an impediment to job creation and economic recovery.

“Enough,” he said. “We’ve had enough.”

Romney won many of the populous suburban counties in the Chicago area, while Santorum took rural areas in the south and west. Romney still remains far from the winning threshold of 1,144 delegates, which means the GOP race will probably continue for weeks.

For Santorum, Illinois was a dismal end to an unhappy week.

Just seven days earlier, the former senator from Pennsylvania had won surprising victories in Mississippi and Alabama. But after that, he stumbled.

He left Illinois — a state where he had a chance to beat Romney — and spent parts of two valuable days campaigning in Puerto Rico, where he had none.

When Tuesday was over, Santorum appeared to have lost in both places. He was already ineligible for 10 of Illinois’ delegates, because he had not filed the correct paperwork.

“We won the areas that conservatives and Republicans populate, and we’re very happy about that,” Santorum told supporters Tuesday night in Gettysburg, Pa.

He said Romney is the wrong man for a crucial time in which the basics of American freedom and free enterprise are at risk: “We don’t need a manager. We need somebody who’s going to pull up government by the roots and throw it out, and do something to liberate the private sector.”

Up next: Louisiana

Santorum’s prospects look better in Louisiana, which will hold its primary on Saturday. But he still appears to have little hope of passing Romney in the race for GOP delegates. On Tuesday, Santorum’s campaign offered its rosiest estimate of the state of that contest — and still had him down by 124 delegates. Outsiders said that was too optimistic.

Illinois was the 27th state to vote this year as an unsettled GOP contest rumbles through places unaccustomed to primary fights. Romney has won 16 of them. Santorum has won nine.

The results in Illinois seemed to further deflate the hopes of former House speaker Newt Gingrich (Ga.), who has won just twice. Going into Tuesday’s vote, Gingrich was already a distant third in the delegate count. In Illinois, he appeared likely to come in fourth, behind Rep. Ron Paul (Tex.).

Exit polls showed a worrisome trend for Gingrich. He scored only in single digits among very conservative and evangelical voters. He also did poorly among voters prioritizing the economy and those looking for a candidate with “the right experience.”

But on Tuesday, as Gingrich faded, those voters went strongly for Romney.

Illinois has importance beyond its delegates, because it looks a whole lot like the GOP electorate in miniature. Preliminary exit polls on Tuesday found that Illinois voters were closely split among “very conservative,” “somewhat conservative” and “moderate or liberal.”

Exit polls showed that 43 percent of Illinois Republican voters described themselves as evangelical or born-again Christians. That was a sharp decline from the number in Mississippi last week — 83 percent — and much closer to the average of 51 percent.

‘The best chance’

In an interview outside Romney’s primary-night party in suburban Schaumburg, Jerry Holmer, 54, said he liked the former governor, even though he considers himself far more conservative than the candidate.

“I think Mitt’s got the best chance of beating President Obama,” said Holmer, a claims risk analyst at a reinsurance company who lives in rural Burlington. “I think he’s straight on with reducing tax rates, and I like the fact that he says he’s got a plan on the first day for getting rid of Obamacare.”

He wasn’t the only one. Among voters who said Romney isn’t conservative enough, exit polls showed, 25 percent voted for him anyway.

Asked about his enthusiasm for Romney, Holmer summed up the candidate’s halting success in three words: “I’m almost enthusiastic.”

The results of the Illinois vote came in slowly on Tuesday night because of an embarrassing error: In many places, the primary ballots wouldn’t fit into the optical scanning machines that were supposed to count them.

In about a quarter of the state’s counties, ballots had to be trimmed to fit or counted by hand.

Earlier Tuesday, Romney visited the Chicago offices of Google. While there, he offered support for the 2013 budget released Tuesday by House Budget Committee Chairman Paul Ryan (R-Wis.).

That budget aims to lower the top tax rate paid by the wealthy while at the same time seeking to wipe out U.S. deficits by 2040. That would be done, in part, by reducing spending on federal benefit programs such as Medicare and Medicaid.

Ryan proposed to limit Medicare spending for future retirees, offering them a set amount with which to purchase private health insurance on newly created federal exchanges.

Retirees could also stay with traditional Medicare under Ryan’s plan, but many could pay more for it.

“I’m very supportive of the Ryan budget plan,” Romney said. “It’s a bold and exciting effort on his part and on the part of the Republicans, and it’s very much consistent with what I put out earlier.”

He added: “I applaud it. It’s an excellent piece of work and very much needed.”

Rucker reported from Illinois. Staff writers Aaron Blake, Sandhya Somashekhar and Rosalind S. Helderman in Washington contributed to this report