By Steve Holland
WASHINGTON (Reuters) - Pressure is mounting on Republican Mitt Romney to win next Tuesday's presidential primary in Illinois, a state considered friendly territory as he seeks to fend off a growing challenge from conservative rival Rick Santorum.
Romney and Santorum have added campaign events in Illinois over the next few days to try to get a leg up in the state, where a Chicago Tribune poll last week gave Romney a narrow lead of 35 percent to Santorum's 31 percent.
The former Massachusetts governor is pouring money into Illinois and sending an ally, New Jersey Governor Chris Christie, to campaign for him there on Friday.
Romney's inability to put Santorum away is causing angst in senior Republican circles and leading to further speculation about the possibility of a contentious convention fight for the presidential nomination when Republicans gather in Tampa, Florida, in late August to formally choose their candidate.
Romney attempted to tamp down such talk on Thursday.
"Look, we're not going to go to a brokered convention," Romney told Fox News. "One or the other of us among the three or four that are running is going to get the delegates necessary to become the nominee."
The 2012 Republican battle has turned into a grinding ground game, with Romney intent on building on his haul of delegates to try to reach the 1,144 needed for the party's presidential nomination as quickly as possible. In Illinois, 69 delegates are at stake.
The Midwestern state's Republicans are relatively moderate and on paper should give Romney a boost after he lost to Santorum in Alabama and Mississippi this week.
Illinois is Democratic President Barack Obama's home state and is expected to vote for Obama in the November 6 presidential election over the Republican challenger.
Romney has 498 delegates nationally, more than double Santorum's 239 and far ahead of Newt Gingrich's 139 and Ron Paul's 69, according to CNN.
MORE THAN JUST MATH
But many Republicans believe he needs more than just delegate math and must produce some big wins in the state-by-state campaign to prove he is a worthy front-runner.
"If Romney wins Illinois, people will say he is sealing the deal as a front-runner candidate," said Republican strategist Ron Bonjean. "If he loses, then they're saying he still has a lot of work to do and he's relying on delegate math and that he may win by the numbers but he also has to win by momentum to generate enthusiasm."
Forces loyal to Romney are pouring money into Illinois. Romney and his backers are believed to be spending more than $4 million there, while Santorum's outside group announced a $310,000 effort.
Word on the street was that Romney came under pressure to win Illinois from some of his financial backers during a day of fundraising in New York on Wednesday.
"The money guys made it clear that you've got to win Illinois," said a veteran Republican strategist, who asked to remain unidentified.
Romney campaign sources cast doubt on the comment and said they were not aware of any such demand. "I don't think anything big happened or I would've heard about it," said one.
Romney has already had big victories this year when his back was against the wall. He won Florida decisively after losing South Carolina to Gingrich, and turned back Santorum in Michigan and Ohio.
"I concede we need to win Illinois," said Republican strategist Charlie Black, an outside adviser to the Romney campaign.
The state on paper is easier to win for Romney than Michigan and Ohio where he scraped out victories against Santorum.
"Illinois if anything is a little more moderate than Michigan or Ohio, with a bigger percentage of suburban voters," Black said.
Romney's campaign insists there is no way Santorum can overtake him for the lead in delegates.
"We have to win 1,144 delegates to get the nomination and we're on a path to do that. No other candidate is," said Romney spokeswoman Andrea Saul. "We're on a path to do that because Mitt's pro-jobs, pro-growth message has resonated across the country."
Some conservatives would like Gingrich to drop out in order to produce a two-man race between Romney and Santorum.
Santorum aides believe the former U.S. senator from Pennsylvania would be able to rally conservatives behind him and defeat Romney if Gingrich were to drop out. But the former U.S. House of Representatives speaker has resisted their subtle entreaties to persuade him to withdraw.
"As far as I know, there is no one pressuring the campaign and Newt is impervious to it," said Bob Walker, a senior Gingrich adviser. "In all honesty, some of it is almost laughable."
(Additional reporting by Alina Selyukh; Editing by Peter Cooney)