Mitt Romney's losses in Alabama and Mississippi on Tuesday offered more stark evidence of a huge stumbling in his quest for the GOP nomination and the White House: his difficulty winning in the evangelical-rich Bible Belt.

With Rick Santorum's victories in Alabama and Mississippi, he and Newt Gingrich have taken all but one Bible Belt state in the primaries.

In Alabama, where self-described white evangelicals made up 75% of Republican voters Tuesday, Romney got 27% of their votes, compared with 35% for Santorum and 32% for Gingrich, according to exit polls.

In Mississippi, where white evangelicals accounted for 80% of the vote, Romney got 29% evangelical support, compared with 35% for Santorum and 32% for Gingrich.

Romney is a Mormon, while Santorum and Gingrich are Catholics. Recent polls show that about half of white evangelicals say Mormonism is not a Christian faith.

"This has nothing to do with religion but with the fact that Romney is only recently pro-life and only recently able to articulate an argument against same-sex marriage," said Richard Land, the public policy chief at the Southern Baptist Convention, the nation's largest evangelical denomination.

"Romney would have had a less difficult pathway to the nomination if he had been more Mormon -- if he had taken the positions of his church on abortion and marriage earlier," Land said.

But exit polls suggest that views about religion played a big role in Tuesday's primaries.

Nearly half of voters in Alabama and Mississippi said the religious beliefs of the candidates mattered a great deal, and Romney fared poorly among those voters, winning 26% of them in Mississippi and just 16% in Alabama.

"Religion is a proxy for trustworthiness of a candidate, especially for white evangelical voters," said Robert P. Jones, a pollster who focuses on religion and values questions. "The thinking goes that if there's a person of faith and good moral character, I can trust them to make decisions even on issues where I don't know where they stand."

Jones notes that the Southern Baptist Convention for years issued materials calling Mormonism a cult. "There's a long history of antipathy," toward Mormons among evangelicals, Jones said, and "that's certainly playing a role in the Deep South."

Santorum, Jones says, has stances on social issues that line up more with rank-and-file evangelicals than with rank-and file Catholics.

In the Bible Belt, Santorum has won primaries in Tennessee, Missouri and Kansas, while Gingrich has won South Carolina and Georgia. Romney won the Virginia primary, but neither Santorum nor Gingrich was on the ballot there.

Still, Romney maintains a big lead among his competitors in terms of delegates.

Because Alabama's 47 delegates and Mississippi's 37 delegates will be awarded proportionally, Romney appeared to maintain his delegate lead and may add to it after more moderate Hawaii's 17 delegates are distributed. A CNN delegate estimate early Wednesday showed Romney with a 489-234 lead over Santorum, giving him a 255-delegate margin.

Mark DeMoss, a senior adviser to the Romney campaign helping with outreach to evangelicals, says he has been discouraging Republican voters from making too big a deal of the candidates' religious faith.

"A candidate's values are critically important, but there are Southern Baptists who don't share my values or political views on many issues, and there are Mormons and Catholics and Jews who do," said DeMoss, a Southern Baptist.