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. ... 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. ... 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. Read more on Newsmax.com: Pressure Mounts on Romney for Illinois Victory Important: Do You Support Pres. Obama's Re-Election? Vote Here Now!