Story highlights Romney spent more than $18 million in January

Newt Gingrich questions President Obama's ability to defend the nation

Rick Santorum attacks Obama for rising gas prices

GOP candidates are preparing for a CNN debate Wednesday in Arizona

Rick Santorum and Newt Gingrich ratcheted up their attacks against President Barack Obama on Monday as they competed for conservative votes ahead of primaries and caucuses in the Republican presidential race.

Campaigning in the perennial swing state of Ohio, Santorum said the president's "radical environmentalist policies" are helping push gas prices to near record highs and are threatening to derail the economic recovery.

He cited Obama's disapproval of the proposed Keystone XL pipeline in the United States and what he characterized as the president's mismanagement of U.S. foreign policy in the Middle East as the cause of "tension" reflected in rising oil prices.

Noting his grandfather once worked in Pennsylvania's coal mines, Santorum also said the White House placed misguided environmental concerns above real economic need.

There's a "lack of real scientific evidence" about global warming, he declared. "Phony studies" on the subject are not "not climate science but political science."

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Gingrich, meanwhile, told a Tulsa, Oklahoma, event that Obama was "incapable of defending the United States" and therefore defeating him in the November election was a "duty of national security."

"The president wants to unilaterally weaken the United States, he wants to cut the aid to Israel for its anti-ballistic missile defense, he refuses to take Iran seriously," Gingrich said.

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

Santorum's emphasis on hot-button social and economic issues comes amid a recent surge in GOP support for the former Pennsylvania senator, who now leads Mitt Romney in a number of national primary polls.

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 Rep. Ron Paul of Texas was close behind at 11%.

The four Republicans, struggling for advantage in what has so far proved an unpredictable nomination fight, are set to meet Wednesday evening in an Arizona debate moderated by CNN's John King.

It will be the last debate before Arizona and Michigan primary voters head to the polls February 28, launching a series of contests including the Super Tuesday showdown in 10 states worth 437 delegates for the nomination.

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.

While next week's contests in Arizona and Michigan have a similar number of delegates at stake with 29 and 30 respectively, most of the attention has been focused on Michigan, a virtual home state for Romney, whose father, George, was once governor there.

Some analysts believe a Romney loss in the state will shatter any remaining consensus that the former Massachusetts governor is the GOP front-runner.

For his part, Gingrich conceded Sunday that losing his home state of Georgia in the March 6 Super Tuesday primaries would "badly" weaken his candidacy.

Appearing on "Fox News Sunday," Gingrich argued that if either of his chief rivals lose their home states -- Romney in Michigan and Santorum in Pennsylvania -- they also would be "badly, badly weakened."

But the former speaker stopped short of saying he would drop out if he fails to win Georgia.

"Given the chaos of this race, I'm not willing to say anything," Gingrich said, adding: "I think it's extraordinarily important to win your home state."

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.

Perhaps emboldened by his increased support, Santorum has launched a series of harsh attacks against Obama and Romney in recent days.

Santorum depicts himself to voters as the only legitimate conservative alternative to the more moderate Romney, and on Monday, he criticized Romney's campaign track record of attack ads on leading rivals.

Stumping for votes in Steubenville, Ohio, Santorum said Americans want a candidate who "doesn't think politics is the equivalent of mud wrestling."

Politics, Santorum said, should be "a higher calling."

He drew the ire of the president's campaign Sunday by declaring the government should never require health care providers to cover fully the cost of prenatal testing such as amniocentesis, which can determine the possibility of Down syndrome or other problems in the fetus.

In particular, amniocentesis "more often than not" results in abortion, said Santorum, who is vehemently anti-abortion, on the CBS program "Face the Nation."

"People have a right to do it, but to have the government force people to provide it free just is a bit loaded," Santorum said in arguing against what he called a mandate in the health care reform legislation passed by Obama and Democrats in 2010.

Lis Smith, a spokeswoman for Obama's campaign, called Santorum's remarks "the latest in a long string of unfortunate comments in the race to the bottom that the Republican presidential primary has become."

"Prenatal screenings are essential to promote the health of both the mother and baby and to ensure safe deliveries," Smith said. "These misinformed and dangerous comments reinforce why women cannot trust any of the Republican candidates for president."

On Saturday in Columbus, Ohio, Santorum appeared to raise questions about Obama's adherence to Bible-based Christian theology in comments that Obama campaign spokesman Robert Gibbs said went "well over the line."

Santorum said then that the president was adhering to "some phony ideal, some phony theology," which he described as "not a theology based on the Bible, a different theology. But no less a theology."

Later asked by reporters about the remark, Santorum said he was trying to say Obama merely holds "different moral values."

Obama has reached a "low in this country's history of oppressing religious freedom that we have never seen before," Santorum added.

Gibbs, appearing Sunday on the ABC program "This Week," said Santorum's comments continued the kind of character attacks that he noted have marked the Republican presidential race so far.

"I think that if you make comments like that, you make comments that are well over the line," Gibbs said. "I think this GOP primary, in many cases ... has been a race to the bottom. We have seen nastiness, divisiveness, ugliness, distortions of opponents' records, of the president's records."