Gingrich, a Catholic, told GOP voters in swing-state Ohio that Obama had declared war on the Catholic Church. He and his GOP rivals have blasted the administration's new regulation requiring church-affiliated employers to cover birth control for their workers. Some Catholics say the rule would force Catholic institutions to violate their religious convictions.
Romney also has criticized the Obama policy, but Gingrich said Romney was no better than Obama on the issue.
"There's been a lot of talk about the Obama administration's attack on the Catholic church," he said at Price Hill Chili restaurant in Cincinnati. "Well, the fact is, Gov. Romney insisted that Catholic hospitals give out abortion pills, against their religious beliefs, when he was governor."
In late 2005, Romney required all Massachusetts hospitals, including Catholic ones, to provide emergency contraception to rape victims. Some Catholics say the so-called morning-after pill is a form of abortion.
In Colorado Springs, Colo., Santorum unleashed a blistering attack on Obama's health care rules.
"Ladies and gentlemen, it's not just your economic rights. It's your freedom of religion. It's your freedom of speech," Santorum, also a devout Catholic.
The sharp rhetoric against the Obama administration's treatment of religious institutions took place as voters in Colorado and Minnesota prepared for GOP caucuses Tuesday night. Romney hoped for more victories following a consecutive wins in Florida and Nevada. Santorum was hoping his weeks of criticism of Romney would catapult him to victory.
"You've got a big caucus tonight," Santorum told supporters in Colorado. "If you look at the polls, today could be a very good day for conservatives."
The outcome of Colorado and Minnesota isn't likely to dramatically change the dynamics of the GOP presidential race. But a Santorum victory in either state could give him a boost - for a day at least - while shining a light on Romney's troubles with conservative voters who long have been skeptical of his candidacy.
Santorum, a former Pennsylvania senator and fierce anti-abortion rights opponent, bypassed Nevada and Florida to essentially camp out in Colorado and Minnesota and spent the past week assailing Romney to lay the groundwork among conservatives who dominate. Santorum has portrayed himself as the only conservative choice in those caucus states.
In recent days, Romney has sensed a Santorum threat and has sought to prove that he, too, has strong conservative stances on social issues, despite a history of reversing himself on abortion and gay rights.
Gingrich and Rep. Ron Paul of Texas also are on ballots in Colorado and Minnesota.
The latest step in the month-old nomination fight comes as Obama's campaign is asking top fundraisers to support a Democratic-leaning outside group that backs his re-election bid and is trying to compete with the tens of millions of dollars collected by Republican-backed outside groups in the presidential race.
It's a reversal for Obama, who has long been opposed to "super" political action committees that can spend unlimited amounts of cash to influence elections. The decision comes at a time when Romney is in a position of strength, after his allied groups successfully beat back challenges by opponents early in the primary season.
Heading into Tuesday, Romney leads in the hunt for delegates to the Republican nominating convention this summer, with 101, while Gingrich has 32. Santorum and Paul trail with 17 and 9 respectively, according to The Associated Press count. Colorado has a total of 36 delegates, including 33 up for grabs while Minnesota has a total of 40 delegates, with 37 at stake. Missouri has no delegates at stake as it holds what amounts to a beauty contest. Instead, Missouri will award its 52 delegates through a system of caucuses and conventions, starting March 17.
In Minnesota, Eric Radtke, 32, was looking for the party's best hope to defeat Obama in the fall and said he planned to vote for Romney because of that.
"Every time I hear him he seems to exude the level of respect for this country that I think it deserves," said Radtke, a telecommunications salesman from Shakopee, Minn.
Terry Groetken couldn't disagree more. The 71-year-old retired optical salesman from Plymouth, Minn., said he would have to "hold my nose" to vote for Romney in November if he wins the nomination because he doesn't trust him to stay consistent on core conservative principles. Groetken planned to vote for Gingrich. "He's a bulldog," Groetken said.
On Monday night, Santorum continued his criticism of Romney, calling him an unfit candidate and warning that Republicans were heading toward defeat if Romney were the nominee.
Romney had criticized Santorum's tenure in the Senate as "not effective" because of his past support for spending on pork-barrel projects. Santorum flatly countered that Romney was "uniquely unqualified."
Elliott reported from Colorado Springs, Colo. Kasie Hunt and Shannon McCaffrey in Colorado and Brian Bakst in St. Paul, Minn., also contributed to this report.