By Dan Gilgoff, CNN.com Religion Editor

(CNN) - Bias against a Mormon presidential candidate hasn’t budged in 45 years, with 18% of Americans saying they would not vote for a well-qualified candidate who happened to be Mormon, according to a Gallup Poll released Thursday.

The survey points up potential challenges for presumptive Republican presidential nominee Mitt Romney, who is vying to be the first Mormon in the White House.

Gallup first asked Americans about support for a Mormon presidential candidate in 1967 when Romney’s father, George Romney, was running for president. That year, 17% of Americans said they would not vote for a well-qualified Mormon for president.

George Romney dropped out of the race after making a gaffe about the Vietnam War, and Richard Nixon won the GOP nomination in 1968.

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“The stability of resistance to a Mormon presidential candidate over the past 45 years is an anomaly,” Gallup editor-in-chief Frank Newport wrote in a survey report, noting that “resistance to a candidate who is black, a woman, or Jewish has declined substantially over the same period of time.”

The survey also found that four in 10 Americans do not know that Romney is Mormon. Gallup found that those who know Romney is a Mormon are also the most likely to back the idea of a Mormon for president.

But the national learning curve on Romney's religion "suggests the possibility that as Romney's faith becomes better known this summer and fall, it could become more of a negative factor," Newport wrote in his report.

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"Those who resist the idea of a Mormon president will in theory become more likely to realize that Romney is a Mormon as the campaign unfolds," Newport wrote.

Bias against a Mormon candidate is significantly higher among Democrats and independents than among Republicans, Gallup found.

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Twenty-four percent of Democrats and 18% of independents said they would not vote for a well-qualified Mormon who was nominated by their party, while 10% of Republicans expressed such opposition.

Resistance to a Mormon candidate was much higher among Americans with lower levels of education, with 23% of those without a high school diploma saying they would not support a well-qualified Mormon. Six percent of those with postgraduate education shared that view.

In his report, Newport said that it’s “unclear how the current level of resistance to the idea of voting for a Mormon presidential candidate will affect the election.”

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“History shows that these types of attitudes in and of themselves are not an impediment to victory,” Newport wrote, citing a 1960 poll that found 21% of Americans would not vote for a well-qualified Catholic candidate for the presidency.

Later that same year, John F. Kennedy won the White House.

The Romney campaign did not immediately respond to a request for comment.

In an interview earlier this year, Southern Baptist leader Richard Land predicted that Romney’s Mormonism would become a bigger political challenge for the candidate - not because of anti-Mormon bias among evangelicals but because of that bias among independents.

Most evangelicals already “know what Mormonism believes and most of them are prepared to vote for Mitt Romney in a general election against Barack Obama in spite of his Mormonism,” said Land, public policy chief for the country’s largest evangelical denomination.

“The 40% of electorate that’s independent, most of them have no idea what Mormons believe,” Land said. “But they will all know what Mormons believe by the general election.”