The Republican presidential frontrunner Mitt Romney has taken a sideswipe at his main rival, Texas governor Rick Perry, in a row over the Mormon religion, an issue that until now had been simmering in the background.

Mormonism is regarded with suspicion by Christian evangelicals, many of whom, according to polls, have cited it as a reason for not voting for Romney.

Perry, who is struggling in the polls, stands to benefit from having the issue become part of the public debate but it could also backfire if it comes to be viewed as part of Texas dirty-tricks politics.

The issue was raised on Friday by the Texas Christian evangelical leader Robert Jeffress, who introduced Perry, a friend, at a conservative conference in Washington DC. Jeffress, speaking later to reporters, described Romney's Mormon religion as a cult and said Romney is not a Christian. "Every true born-again follower of Christ ought to embrace a Christian over a non-Christian," said Jeffress.

Romney, who usually avoids discussing religion, hit back on Saturday, though indirectly. Although Romney said his words were directed at a radio host with a reputation for intolerance due to speak later at the conference, his comments could be viewed too as a rebuke to the Perry camp.

"Poisonous language doesn't advance our cause. It's never softened a single heart nor changed a single mind," Romney said. "The blessings of faith carry the responsibility of civil and respectful debate. The task before us is to focus on the conservative beliefs and the values that unite us. Let no agenda narrow our vision or drive us apart."

At the start of his speech, Romney praised the speaker that preceded him, former member of the Reagan cabinet Bill Bennett, saying: "And speaking of hitting it out of the park, how about that Bill Bennett? Isn't he something else?"

Bennett had openly criticised Jeffress, accusing him of "bigotry" and saying he had damaged Perry's campaign.

The battle for the Republican nomination to take on Barack Obama in November next year has essentially come down to Romney versus Perry, both of whom are attracting millions of dollars in support. Herman Cain is enjoying a surge in polls that puts him neck-and-neck with Romney, but his lack of major donors raises questions about his staying power.

Cain's mixture of rightwing policies and sense of humour made him a conference favourite.

In a straw poll of the strongly social conservative audience, the maverick Ron Paul won easily with 37%. Paul's supporters tend to be young and committed and have made a speciality out of organising for such polls. Cain, in a further sign of his sudden popularity, came second with 23%; Rick Santorum, a strong social conservative took 16%; Perry and Michele Bachmann 8% each; Romney 4%; Newt Gingrich 3% and Jon Huntsman 0%.

Perry joined the presidential race in August and quickly established a clear poll lead over Romney but he has since seen that collapse after a string of poor performances in televised debates.

Perry, in answer to a question from a reporter, said he did not share Jeffress's view that Mormonism is a cult. His campaign team said that though they had signed off on Jeffress introducing the candidate, he had been recommended by others.