An Iowa woman who supported Pete Buttigieg for president in the tumultuous US Democratic caucus asked to change her vote when she learned the candidate is gay.

She cited her religious beliefs for doing so.

"Are you saying that he has a same-sex partner? Are you kidding?" the woman, wearing a "Pete 2020" sticker, asked a caucus organizer, known as a precinct captain, in rural Iowa on Monday evening. "Well then I don't want anybody like that in the White House. So can I have my card back?"

The exchange was caught on a video that quickly went viral online on Tuesday as Buttigieg took a shock lead in the caucuses, according to partial delayed results.

The Buttigieg precinct captain in Cresco, Iowa earned praise for her measured, compassionate reaction in which she sought to reassure the unidentified woman that a candidate's sexuality was not a concern.

"I would like you to just dig deep inside and think, should it matter if it's a woman or if it's a man or if they're heterosexual or homosexual, if you believe in what they say?" says the caucus official, identified online as Nikki Heever. "Well he'd better read the Bible," replied the woman.

Heever told her that Buttigieg did, and that she respected her viewpoint. "I just ask you to look inside your heart, because you sound like a Christian woman to me and I'm a Christian woman, and my God wants me to love everybody," Heever said.

Buttigieg in a speech on Tuesday made a rare, though implicit, reference to his sexuality. His strong performance was a boost "for a kid somewhere in a community wondering if he belongs, or she belongs or they belong in their own family," he said with a catch in his voice.

Polls show mixed results regarding whether Americans would be comfortable with an openly gay president. Internal focus group data obtained by the McClatchy news organization last year showed that many black voters, an important Democratic constituency, could view Buttigieg's sexuality as a barrier.

Buttigieg and Vermont Senator Bernie Sanders led the process to choose state Democrats' pick to challenge Republican Donald Trump in November's election. Buttigieg, a polyglot Rhodes scholar, is the former mayor of South Bend, Indiana, and served in Afghanistan as a US Navy intelligence officer.

Last year, his same-sex marriage to Chasten Glezman was widely covered in US media as Americans mulled the prospect of a same-sex couple in the White House.