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Gerald Herbert, Associated Press
Republican presidential candidate, former Massachusetts Gov. Mitt Romney speaks at a campaign rally in Loveland, Colo., Tuesday, Feb. 7, 2012.

LOVELAND, Colo. — Republican presidential candidate Mitt Romney railed against President Barack Obama's "assault on religion" Tuesday, part of an intensified effort to win over social conservatives in GOP caucuses in Colorado and Minnesota.

Romney, who previously supported abortion rights and is now an opponent, has struggled to convince some cultural conservatives that he's conservative enough. Rival Rick Santorum, long an outspoken figure on social issues, hopes those voters will help him finish strong on Tuesday.

"Remarkably under this president's administration there is an assault on religion — an assault on the conviction and religious beliefs on members of our society," Romney told supporters in Loveland, his only campaign event before the caucus results are known.

He cited the Obama administration's recent decision to require Catholic organizations to provide contraceptive aids in some circumstances. Romney called the ruling "a real blow ... to our friends in the Catholic faith" and likened "morning-after pills" to "abortive pills."

"This kind of assault on religion will end if I'm president of the United States," he pledged.

But as Romney stepped up his emphasis on religious freedom, Santorum and rival Newt Gingrich intensified their criticism of Romney on that issue.

Gingrich, a Catholic, told voters in Ohio that Obama had declared war on the Catholic Church, and that 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," Gingrich said at a 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 an op-ed published Tuesday in Politico, Santorum seized on Romney's 2005 decision.

"He said then that he believed 'in his heart of hearts' that receiving these contraceptives — free of charge — trumped employees' religious consciences," said Santorum, a Catholic. "Now, a few years later and running for president, his heart is strategically aligned with religious voters opposing this federal mandate."

Romney's focus came as his campaign prepared for a weak showing Tuesday. Campaign political director Rich Beeson issued a memo to reporters that said Romney won't win every contest. He has lost two of the contests held thus far.

"John McCain lost 19 states in 2008, and we expect our opponents will notch a few wins, too," Beeson wrote. "But unlike the other candidates, our campaign has the resources and organization to keep winning over the long run."