Editor's note: David Frum, resident fellow at the American Enterprise Institute, was special assistant to President George W. Bush in 2001-2. He is the author of six books, including "Comeback: Conservatism That Can Win Again" and the editor of frumforum.com

(CNN) -- Catholic, Orthodox and Evangelical Christian leaders last week issued a bold political statement. They intended to target the Obama administration. Inadvertently, they may have also hit probable Republican presidential candidate, Mitt Romney.

Mitt Romney ought to rank atop the Republican candidates for president in 2012. He finished second in votes cast in the primaries of 2008. He is a candidate with immense private-sector economic expertise in a time of urgent economic debate. But Romney has a political problem: his Mormon religious faith. A Gallup survey in December 2007 found that 18 percent of Republicans would not vote for a Mormon for president.

Romney has worked hard to persuade Republicans to think again. In the 2008 cycle, many conservative Christians showed support for his candidacy. But the important new statement by Christian leaders suggests that Romney may face even greater religious resistance in 2012.

The Manhattan Declaration -- that is the statement's name -- offers an ominous assessment of the Christian condition in Barack Obama's America. It warns that the administration and its supporters will "trample upon the freedom of others to express their religious and moral commitments." It worries that Christians may soon face outright persecution at the hands of government authorities:

"[W]e remember with reverence those believers who sacrificed their lives by remaining in Roman cities to tend the sick and dying during the plagues, and who died bravely in the coliseums rather than deny their Lord. ... Like those who have gone before us in the faith, Christians today are called to proclaim the Gospel of costly grace."

In the face of these looming threats, more than 125 signatories pledged themselves to outright civil disobedience.

"[W]e will not comply with any edict that purports to compel our institutions to participate in abortions, embryo-destructive research, assisted suicide and euthanasia, or any other anti-life act; nor will we bend to any rule purporting to force us to bless immoral sexual partnerships, treat them as marriages or the equivalent, or refrain from proclaiming the truth, as we know it, about morality and immorality and marriage and the family."

Now notice something curious: not one of the initial publicly identified signatories of the Manhattan Declaration is Mormon.

Through the cultural conflicts of the past decade, Mormons and the Mormon church have played a decisive role. The church itself gave $190,000 to the fight to repeal same-sex marriage in California. Individual church members many millions more. (McClatchy newspapers have quoted estimates as big as $20 million, although that seems improbably high.)

That degree of commitment might seem to entitle you to a seat at the table. But no. The framers of the Manhattan Declaration say they "act together in obedience to the one true God, the triune God." Mormons do not accept the concept of God as three-in-one.

Conceded: Every religious grouping sets its own boundaries and definitions. Sikhs revere Muhammad as a prophet, but they are not Muslims. Jews for Jesus sometimes keep kosher, yet they are not regarded as Jews by most religious authorities. Christians are of course entitled to decide for themselves whether they will accept Mormons as "part of us."

But when it comes time to act in politics, it's the American way to leave sectarian tests behind. The most important social conservative group of the 1970s, Jerry Falwell's Moral Majority, made room for Mormons and Jews too.

Now that tradition is to be departed from. The next wave of social conservatism is presenting itself as a particularly Christian cause, with Christian defined in a way that would exclude not only Mitt Romney, but also the man who created Tiny Tim and Ebenezer Scrooge. (Charles Dickens was a Unitarian, not a Trinitarian.) For that matter, neither George Washington, nor John Adams, nor Thomas Jefferson, nor Abraham Lincoln was a believer in the Trinitarian God of the Manhattan Declaration.

Now here's the question. If this is a time when Christians must act as Christians together with other Christians -- and if Mormons do not qualify -- how can such Christians accept a Mormon like Mitt Romney as their political leader?

Evangelical leaders in 2008 tried to sell the idea that a man could be religiously repugnant yet politically acceptable. The influential Charles Colson famously cited the reputed remark of Martin Luther: Better to be governed by a wise Turk than a foolish Christian.

Yet just as Gallup predicted at the beginning of the 2008 cycle, the pro-Romney sympathies of some evangelical leaders did not translate into pro-Romney primary votes. Romney not only failed to win a single southern state in the Republican primaries, but he did not even finish second anywhere south of the Mason-Dixon line.

The exit polls strongly suggest the power of unease over Romney's faith. In Alabama, 78 percent of GOP primary voters described themselves as evangelicals. Half of them voted for Mike Huckabee, and that boost helped Huckabee win the state. In Georgia, 64 percent of Republicans identify as evangelicals. They voted 41 percent for Huckabee, 28 percent for Romney.

Romney did best in the secular Northeast (he won Massachusetts and Maine, and finished second in New Hampshire, New Jersey and Connecticut) and the West (he won Wyoming, Colorado and Nevada, and finished second in California and Arizona).

In the end, while Romney won almost 400,000 more votes than Huckabee and three more states, it was Huckabee who finished second in the delegate count: 270 vs. 140.

Since 2008, Huckabee's strength has grown inside the GOP. Romney's has sagged. Romney's 56 percent favorability rating among Republicans badly trails Huckabee's 72 percent.

Like many Republicans, I have many questions about Mitt Romney's bid for party leadership. They all relate to his public record and his civic convictions. I don't share his religious views. But is it not disturbing that in the United States in the 21st century a man of unquestioned personal rectitude should feel compelled to say, as Romney said in December 2007:

"If I am fortunate to become your president, I will serve no one religion, no one group, no one cause, and no one interest. A president must serve only the common cause of the people of the United States.

"There are some for whom these commitments are not enough. They would prefer it if I would simply distance myself from my religion, say that it is more a tradition than my personal conviction, or disavow one or another of its precepts. That I will not do. I believe in my Mormon faith and I endeavor to live by it. My faith is the faith of my fathers -- I will be true to them and to my beliefs."

Mormon America has provided leadership and support for conservative politics out of all proportion to its numbers. If there's a test for conservative identity that excludes Mormons, it's not a good test. And if conservatism has shrunk too small to contain conservative Mormons, it is not only Mormons who will search for something bigger.

The opinions expressed in this commentary are solely those of David Frum.