When a pair of Mormon missionaries knocked at the door of Jerry Pierce's home in a north Dallas suburb last month, he marshalled his arguments and stood his ground.

"I look forward to encounters like that. I like to talk to them about the nature of Christ and who Jesus is," said Pierce, a staunch Southern Baptist, the biggest Protestant denomination in the United States.

Mitt Romney, a Mormon, is running into similar resistance as he tries to win over Southern Baptists and other evangelical Protestants in the race for the Republican Party's nomination for the 2008 U.S. presidential election.

Romney will need the support of this traditional Republican base if he is to take on former New York mayor Rudy Giuliani, who is running strongly in opinion polls despite his three marriages and a pro-abortion position that is anathema to many Republicans.

The reason Romney faces difficulties with Southern Baptists, according to many experts, is his Mormon faith. Not only do many Southern Baptists regard the Mormon church as a cult, they also regard it as a competitor that is winning -- or poaching -- converts from among the evangelical flock.

"There are now more Mormons that used to be Southern Baptist than any other denomination," said Dr. Richard Land, president of the Ethics & Religious Liberty Commission of the Southern Baptist Convention, a 16-million strong group.

"As a consequence, Southern Baptists and other evangelicals have taught their people what Mormons believe and why it's beyond the boundaries of the Christian faith, to inoculate them against those Mormon missionaries," he told Reuters.

This is no small matter for people who take their faith as seriously as Southern Baptists do, and to counter the perceived threat they teach their members in Sunday School to be ready for that knock at the door and be wary of Mormon missionaries.

Romney himself did missionary work overseas.

Some say a Mormon in the White House would help the faith -- founded in 1830 in New York state by Joseph Smith and still struggling with the legacy of polygamy -- become more accepted. This is a dim prospect for evangelical leaders, who see themselves competing with the religion, literally, for souls.

"Many evangelicals do not want to see Mormonism mainstreamed," said Matthew Wilson, a political scientist at Southern Methodist University in Dallas.

For Romney, the stakes are high. He is casting himself as a social conservative family man opposed to abortion and gay marriage, in a bid to win over white evangelical Protestants, who account for about a third of the Republican electorate by some estimates.

A recent poll by the Pew Research Center found he only had the support of about 10 percent of white Republican and Republican-leaning evangelicals.

"There are a lot of conservative Christians who are going to look at the Mormon thing and say, 'Wait a minute, he may be conservative but he's a Mormon,' and they're not going to go there," said Steve Swofford, a pastor in the city of Rockwall, near Dallas, and former president of the Southern Baptists of Texas Convention.

Romney has managed to make some headway with evangelicals who have yet to unite around a single Republican candidate.

He has received support from politically active evangelical leaders such as Paul Weyrich and there is a small group of bloggers called "Evangelicals for Mitt."

On the other hand, there is no group called "Mormons for Mitt" and Romney plays down his faith.

Many Americans associate Mormonism with polygamy, once a central tenet but now practiced by only about 40,000 renegades. One of those renegades, polygamist sect leader Warren Jeffs, was sentenced to 10 years to life on Nov. 20 for forcing a 14-year-old girl to marry her cousin.

Evangelical Protestants tend to be more aware than most that the Mormon church now opposes polygamy, but they have other bones of contention, such as Mormon efforts to recruit their members.

NUMBERS AND GROWTH

There is no hard data -- the Mormon church does not crunch numbers on the previous faiths of its converts -- but there is evidence pointing to Mormon inroads among Southern Baptists.

The Mormon faith is growing faster than almost any other. In 1963, its membership stood at two million but now is close to 13 million with over half outside of the United States.

"The church of course is growing everywhere, now somewhat faster overseas than in America. But we do not as a matter of policy attack other churches," said Mike Otterson, a spokesman for the Mormon church in Salt Lake City.

In America some of its fastest growth has been in Southern Baptist strongholds, notably the South, according to data provided by the Mormon church.

In the United States overall its membership grew by 3.2 percent from 2004 to 2006. But in 13 states of the South its numbers grew over the same period by 5.3 percent.

Evangelical soil can be fertile ground for Mormon growth.

Mormons and Southern Baptists take similar conservative stances on social issues and tend to vote Republican, so their cultural and political outlooks are not really in conflict.

"Some Southern Baptists will live near Mormon communities functioning at their best, where they will see in practice the kind of family-oriented, sober, diligent, and disciplined lives that Southern Baptists preach but do not always display," said Mark Noll of the University of Notre Dame.

Noll, a leading evangelical historian, also said the theological underpinnings of the faiths had some similarities.

"Southern Baptists maintain a vigorous doctrine of divine revelation. That latter belief is not too far from the Mormon belief that God spoke to and through Joseph Smith," he said.