(CNN) — On the eve of the last debate before crucial primaries, Republican presidential challengers Rick Santorum and Newt Gingrich battled for conservative support by targeting President Barack Obama in increasingly strident attacks Tuesday.

Santorum told a crowd in Arizona, which will hold its primary with Michigan on February 28, that Obama and liberals consider the U.S. Constitution an out-of-date document for modern times.

Citing the reported comment by Supreme Court Justice Ruth Bader Ginsburg that the South African Constitution is a better modern model in some respects, Santorum said such thinking amounted to disrespect on the political left for America’s founding document.

Obama believes “that this document has lived past its expiration date and that we need something new, and since it is so hard to change, these old men that put this together made it really hard for it to change, so we’ll just find out a new way,” Santorum said. “We’ll use the courts or in the case of President Obama we’ll just run roughshod over the Constitution and do whatever we want to do.”

Gingrich, meanwhile, said in an interview on the CBS program “The Early Morning” that Obama’s policies pander to Islamic countries and the federal government refuses to “talk accurately about radical Islam.”

“I think it’s dangerous to America,” Gingrich said a day after he accused Obama of being incapable of defending the country.

Gingrich also criticized Obama over what he called “outrageously anti-American energy” policies such as the failure to grant approval for the Keystone XL oil pipeline from Canada’s tar sand production and the administration’s push for more efficient cars.

At a later campaign event, Gingrich outlined his energy policy by calling for opening up federal holdings on land and at sea for development, as well as writing off the cost of new equipment in the first year.

Both Santorum and Gingrich have ratched up their rhetoric in recent days as the topsy-turvy campaign endured another shift, with Santorum surging to the top of the polls while Gingrich has slipped well back.

The two are considered the conservative challengers to the more moderate Mitt Romney, a consistent top-tier candidate who has received solid but, so far, limited support. The fourth candidate, Texas Rep. Ron Paul, also has the backing of a devoted, but limited, following for his libertarian views.

The 8 p.m. ET debate Wednesday, to be broadcast on CNN, will be the last face-to-face showdown before the Arizona and Michigan primaries and subsequent contests including the Super Tuesday showdown on March 6 when 10 states and 437 delegates will be at stake.

A new CNN/Time/ORC International poll Tuesday showed Romney and Santorum in a statistical tie at the top among likely voters in next week’s Arizona primary, with Gingrich well back in third place and Paul trailing.

Arizona officials said that more than 200,000 early votes had been cast a week before the primary, a dynamic expected to favor Romney because the early balloting began before Santorum’s surge began.

Randy Pullen, a former chairman of the Arizona Republican Party and a prominent Romney supporter, said the expected Romney lead in early voting should overcome Santorum’s rising popularity.

“On election day, I think Rick Santorum is going to win. But I think early balloting is going to Mitt. I think enough votes have already been cast,” Pullen said.

Romney campaigned Tuesday in Michigan, which is considered his home turf because he grew up there and his father George served as governor back in the 1960s and ’70s. Santorum’s surge has raised the previously unimaginable possibility that Romney could lose Michigan, a result that would launch questions about whether Romney can gain any conservative backing to win the nomination.

Pullen said a Romney loss in Michigan would be a “devastating” defeat.

“That would be very difficult to explain — how you lose your home state,” Pullen said, though he added a loss in Michigan could be offset by a win in Arizona.

At a town-hall-style meeting Tuesday in Shelby Township, Romney tackled a host of social issues that he has generally avoided at more scripted events, but have been brought to the forefront by controversial comments by Santorum in recent days.

Seeking to establish his conservative credentials in the face of Santorum’s energized campaign, Romney declared he was anti-abortion and would cut off funding for Planned Parenthood if elected. He added that his running mate would be pro-life and “conservative to the core” if he gets the GOP nomination.

Romney also accused Obama of following a “secular agenda” and said a Romney administration would promote religious freedom.

“You expect the president of the United States to be sensitive to that freedom and protect it and unfortunately, perhaps because of the people the president hangs around with, and their agenda, their secular agenda, they have fought against religion,” Romney said.

In a rare reference to his Mormon faith, Romney continued by saying: “I can assure you, as someone who has understood very personally the significance of religious tolerance and religious freedom and the right to one’s own conscience, I will make sure that we never again attack religious liberty in the United States of America.”

Romney also took a shot at Santorum, responding to a question from the crowd by saying the former Pennsylvania senator had yet to undergo the rigorous scrutiny that comes with being a front-running candidate.

“Rick Santorum voted to raise the debt ceiling five times without getting compensating reductions in spending,” Romney said. “The fact that he continues to defend earmarks, including his $500,000 earmark to the Pittsburgh zoo for a polar bear exhibit — I don’t think that is consistent with the principles of conservatism. I don’t think Rick Santorum’s track record is that of a fiscal conservative.”

Paul’s campaign also questioned Santorum’s record as fiscal conservative in a new television commercial in Michigan.

“Is this dude serious? Fiscal conservative? Really?” asks the narrator in the ad. “Santorum voted to raise the debt ceiling five times, doubled the size of the Department of Education, then supported the biggest entitlement expansion since the 1960s. Not groovy. Santorum voted to send billions of our tax dollars to dictators in North Korea and Egypt, and even hooked Planned Parenthood up with a few million bucks. Rick Santorum a fiscal conservative? Fake.”

The Santorum campaign was quick to respond, with national communications director Hogan Gidley telling CNN that “for all of Ron Paul’s blustering about conservatism, the bottom line is that he’s been in Congress for decades and has not had a single accomplishment to forward the cause of conservatism. Not one.”

Santorum later told a Maricopa County GOP lunch that “I voted for smaller government, lower taxes, less regulation — the things that we need desperately in this country.”

“You see all these commercials — Rick Santorum is a big spender — but they never once mention, talk about how I voted for any increase in the appropriation bills. Why? Because I never did,” Santorum said. “I voted to cut appropriation bills. They never talk about I voted for a tax increase why? because I never did in 16 years of public life.”

The heightened rhetoric reflected mounting stakes in the campaign to choose a Republican nominee to face Obama in November.

Gallup’s daily tracking poll Monday showed Santorum leading Romney 36% to 26% among Republicans nationwide. The new numbers represented a six-point drop for Romney since last week, when the former Massachusetts governor was statistically tied with Santorum, who rose by five points in the same period.

Gingrich, a former House speaker, has seen his poll numbers decline over the past month since his only primary win in South Carolina on January 21.

According to Gallup, Gingrich came in third place in Monday’s poll with 13%, and Paul close behind at 11%.

Romney’s once presumptive march to the nomination has come into question since Santorum swept contests in Colorado, Minnesota and Missouri on February 7. For Romney, the question is why he can’t increase his always solid but never dominating support as the primary campaign moves across the country.

Romney’s campaign said Monday it raised $6.5 million in January, while Federal Election Commission findings showed it significantly depleted resources by burning through $18.7 million during the month.

According to the documents, Romney finished January with $7.7 million on hand and no debt after starting the month with $19.9 million on hand.

Gingrich finished January with almost $1.8 million cash on hand and owing nearly as much in debt, his campaign disclosed Monday. He raised nearly $5.6 million during the month, according to financial reports filed with the Federal Election Commission.

Santorum’s campaign brought in $4.5 million in January, more than the candidate spent over the course of a month that saw his poll numbers rise and his national profile expand.

According to filings with the Federal Election Commission, the former Pennsylvania senator spent $3.3 million in January, and ended the month with nearly $1.5 million cash on hand.

Paul raised about the same amount as Santorum and has $1.6 million cash, according to his campaign and filing documents.

CNN’s Shawna Shepherd, Marlena Baldacci, Paul Steinhauser, Gregory Wallace, Peter Hamby and Rachel Streitfeld contributed to this report.