(CNN) -- Mitt Romney and Rick Santorum launched a final campaign blitz across Illinois on Monday, the eve of their showdown in the Land of Lincoln primary that could cement Romney's front-running status or boost Santorum's conservative-backed surge in the Republican presidential race.
Tuesday's Illinois primary offers 54 delegates and follows a strong Romney victory in Puerto Rico, where the former Massachusetts governor picked up all 20 delegates at stake on Sunday by winning more than 80% of the vote.
CNN's latest delegate estimates show Romney with 519 delegates to Santorum's 239. Former House Speaker Newt Gingrich has 138 delegates and Texas Rep. Ron Paul, the libertarian champion, has 69. To secure the nomination, 1,144 delegates are needed. In the Illinois contest, delegates will be awarded proportionately.
Despite his growing lead in the delegate count, Romney still faces questions about his ability to win support from the Republican conservative base and needs a victory in Illinois to answer doubters.
An American Research Group poll released Monday indicated Romney had a solid lead over Santorum.
According to the survey conducted over the weekend, 44% of likely Republican primary voters favored Romney compared with 30% for Santorum, 13% for Gingrich and 8% for Paul. The sampling error was plus or minus 4%.
Santorum already has won contests in neighboring Minnesota, Iowa and Missouri. Capturing Illinois would continue his momentum in the region.
The former Pennsylvania senator, who has emphasized his campaign's humble beginning against the better-financed and better-organized Romney, said over the weekend that winning Illinois would signal ultimate triumph.
"This is a pledge," Santorum said in Effingham. "If we're able to come out of Illinois with a huge or surprise win, I guarantee you, I guarantee that we will win this nomination."
On Monday, Santorum told the CBS "This Morning" program from Rockville, Illinois, that having four candidates competing for the GOP nomination could prevent any of them from securing the nomination before the August convention.
"I think it's going to be very difficult as this goes on for anybody to get to that magic number" of 1,144 delegates, Santorum said.
Santorum and Gingrich are competing for conservative support against the more moderate Romney, and Santorum told WLS radio Monday that Gingrich "has just taken the conservative vote and divided it."
"And by and large, Gov. Romney can't get above 35% of the vote anywhere," Santorum added. "That shows you he hasn't been able to close the deal in spite of enormous money advantages as you see on TV here in Illinois, and the robocalls and all the other establishment media being in his corner -- he can't seal the deal."
Romney kicked off his Monday campaigning in Springfield, saying at a pancake breakfast that he has the economic gravitas to bring growth and job creation, unlike an "economic lightweight" like President Barack Obama.
He didn't mention Santorum by name, but his rhetoric was the same as weekend criticism that also characterized his main foe as an economic lightweight.
Later, Romney used a speech at the University of Chicago's Harris School of Public Policy, where Obama once served on the advisory board, to depict the presidential contest as a battle with Obama over the economy.
Romney made no mention of his GOP foes, instead focusing on what he called Obama's "assault on freedom" through stifling business regulations and high taxes.
"This November, we face a defining decision," Romney told an audience largely of college students. "Our choice will not be one of party or personality. This election will be about principle. Our economic freedom will be on the ballot. And I intend to offer the American people a clear choice."
According to Romney, Obama's "heavy hand" has cast a pall of uncertainty over the economy, preventing entrepreneurs from investing or starting small businesses. Inventors such as Thomas Edison, the Wright brothers and Steve Jobs would have never have succeeded in the current economy, Romney claimed.
Santorum later mocked Romney for promoting "freedom" while also having passed a health insurance mandate in Massachusetts.
"Let's just be brutally honest about it," Santorum said at an event in Dixon, Illinois. "There is one candidate in this race who can never make this race about freedom because he simply abandoned freedom when he was governor of Massachusetts and he abandoned it when he promoted" federal health care reform in 2009.
Obama advisers, meanwhile, told reporters in a conference call that independent tax analysts had concluded that Romney's tax plan will increase deficits by $5 trillion over the next 10 years.
They also challenged Romney's repeated claim that the regulatory climate is slowing economic production, with Princeton economics professor Cecilia Rouse -- a former member of the president's Council of Economic Advisers -- saying there was "not actually any evidence that regulatory burden actually has a meaningful impact on employment or the economy now."
On Sunday, Romney framed his win in Puerto Rico as indicative of the territory's desire for a candidate that "most represents their feelings."
"Those people who don't think that Latinos will vote for a Republican need to take a look in Puerto Rico," Romney said, noting that the territory's governor and its legislative leaders are conservative. "Hispanic voters are going to vote for Republicans if we stand for something -- conservative principles that bring growth and good jobs and rising home values. That's how we're going to win, and we're going to get Latino voters to help us out."
Romney had entered the contest in Puerto Rico as the favorite. He was largely backed by the island government's political establishment, including Gov. Luis Fortuno, who campaigned with Romney last week.
Santorum created a small political firestorm on the island in the days leading up to the primary when he said English should be the principal language in Puerto Rico before it could gain statehood. Puerto Rico will vote on a statehood referendum in November.
After arriving in Puerto Rico on Friday, Romney said he would have "no preconditions" on language for Puerto Rico to gain statehood, though during a CNN debate in January he said English should be the nation's official language.
Santorum immediately hit back, accusing Romney of flip-flopping.
Romney fired back that English has been the official language of the government in Puerto Rico for more than 100 years.
The heated, see-saw allegations between the two candidates have marked much of the race for the GOP nomination, which Sen. John McCain, R-Arizona, called "the nastiest I've ever seen" during an appearance Sunday on NBC's "Meet the Press."
Asked about McCain's comment, Santorum told CBS on Monday that Romney has been the main practitioner of the kind of negative campaigning McCain referenced, in particular advertising funded by super PACs.
"It's not Rick Santorum running all the negative ads or even Newt Gingrich," Santorum said. "It's Mitt Romney who has systematically just gone out there and run a negative campaign, has had no positive vision for this country and spends billionaire dollars to tear down every single opponent that's in his path."
CNN's Peter Hamby, Paul Steinhauser, Jim Acosta, John Helton, Ashley Killough, Rachel Streitfeld and Shannon Travis and Candy Crowley contributed to this report.