Donald Trump has called on White House rival Hillary Clinton to fire an aide involved in leaked emails in which conservative Catholics were disparaged.
The comments were made in an alleged April 2011 exchange between Clinton communications director Jennifer Palmieri and a think tank fellow.
Her then-colleague appears to mock media mogul Rupert Murdoch for bringing up his children as Catholics.
Republican House Speaker Paul Ryan said the comments were "staggering".
The exchange was revealed this week among thousands of emails disclosed by WikiLeaks.
The messages detail a conversation between Ms Palmieri and John Halpin, a senior fellow at the Center for American Progress, a liberal think tank with White House links.
Mr Halpin, who is himself Catholic, wrote: "Friggin' (Rupert) Murdoch baptised his kids in Jordan where John the Baptist baptised Jesus."
He adds that "the most powerful elements of the conservative movement are all Catholic".
Mr Halpin also described their positions as "an amazing bastardisation of the faith".
"They must be attracted to the systematic thought and severely backwards gender relations and must be totally unaware of Christian democracy," he added.
Ms Palmieri, who was at the think tank at the time, responded that Catholicism "is the most socially acceptable politically conservative religion".
"Their rich friends wouldn't understand if they became evangelicals," she said.
Clinton campaign chairman John Podesta - whose emails were hacked - was included in the email chain, but did not respond.
He has said the FBI is investigating whether Russia was behind the cyber-breach of his system.
Campaigning on Wednesday in the battleground state of Florida, Mr Trump said Mrs Clinton's team had been "viciously attacking Catholics and evangelicals".
The Republican nominee added: "Anybody of religion, I really think you have to vote for Donald Trump."