WASHINGTON -- Rick Santorum, the former Pennsylvania senator who has suddenly become Mitt Romney's top challenger for the Republican presidential nomination, raised a provocative question about Romney's Mormon faith in a 2007 newspaper column:

Would the potential attraction to Mormonism by simply having a Mormon in the White House threaten traditional Christianity by leading more Americans to a church that some Christians believe misleadingly calls itself Christian, is an active missionary church, and a dangerous cult?

His next paragraph began: "Assume for the sake of argument that there are valid considerations."

The column was Santorum's response to Romney's famous "Faith in America" address, in which the former Massachusetts governor tried to reassure evangelical Christians and other religious voters that he shared their values. Santorum's ultimate verdict on Romney was more or less positive: "He should be a viable choice for voters whose faith matters to them," Santorum wrote.

But throughout the piece, Santorum drew distinctions between Mormonism and Christianity that others have avoided lest they seem overly inflammatory in a political context.

For instance, Santorum wrote of Romney's speech, "He tried to address the questions by discussing Jesus, suggesting that the specific theological tenets of Mormonism are not in any important respect different from those of traditional Christianity. I disagree."

Romney was the highest-ranking lay leader of the Church of Jesus Christ of Latter-Day Saints in Boston. And skepticism if not fear of his deep Mormon faith is widely considered to be a significant and ongoing problem for him among evangelical voters in particular.

A top Romney adviser recently told The Huffington Post's Howard Fineman that "the Mormon factor" is "just a fact of life that we know we have to deal with."

Santorum's come-from-way-behind showing in the Iowa caucuses on Tuesday could be attributed in no small part to the fact that several prominent state preachers suddenly endorsed him when all the other "anybody but Romney" candidates had crashed and burned, one after the other.

About 60 percent of Iowa's GOP caucus-goers self-identify as evangelical, and according to CNN entrance polling suggets that Santorum got 34 percent of their votes, compared to 14 percent for Romney.

The undercurrent of anti-Mormon concern has only occasionally risen to the surface of the political discourse. One notable exception came in October, when a prominent Texas pastor who introduced Texas Gov. Rick Perry at the Values Voters Summit called Mormonism a "cult."

In an October interview on Fox News, Santorum was asked if he thinks Mormonism is a cult, and he replied, "No, I don't." He added, "I'm not an expert on Mormonism. All I know is that every Mormon I know is a good and decent person, has great moral values and, by and large, with the exception of Harry Reid, by and large, pretty consistent in the values that I share and that things I want to see happen to this country. And that's what he should be judged on."

Santorum has made his fierce adherence to conservative Roman Catholic "family values" a keystone of his campaign, most recently saying he believes states should have the right to outlaw birth control and sodomy without the interference of the Supreme Court and that gay marriage could doom the United States.

In his 2007 column, Santorum made it clear he thinks matters of faith are fair game. He quoted Romney as saying that "a person should not be rejected ... because of his faith." Then he wrote, "His supporters say it is akin to rejecting a Barack Obama because he is black. But Obama was born black; Romney is a Mormon because he accepts the beliefs of the Mormon faith. This permits us, therefore, to make inferences about his judgment and character, good or bad."