The Republican presidential hopeful, Rick Santorum, spoke at a Messianic Jews convention in 2010, it emerged this week.
Mr Santorum, who remains the main obstacle to Mitt Romney's chances of the party's nomination, is known for his conservative views on issues such as abortion, homosexuality and birth control, inspired by his devout Catholicism.
He has strong support among the party's core voters, but the revelation that he was paid to address a controversial Messianic Christian group is unlikely to boost his chances in the upcoming New York and California primaries, where a total of 267 delegates are at stake.
According to Politico, Mr Santorum accepted £3,700 to speak at an event hosted by the Messianic Jewish Alliance of America, a group that has been condemned by the Anti-Defamation League for teaching that "Jews should convert to Christianity".
Abraham Foxman, the ADL national director, said: "Political figures are free to raise money from whomever they want, so long as they disclose it, but considering the role Rick Santorum sees for religion in public life, it is very distressing that he would appear on the platform of a group that teaches that Jews should convert to Christianity.
"His decision to appear before a group of Messianic Jews was insensitive and offensive."
It emerged earlier this year that Mr Santorum 's campaign was being managed by a former senior aide to Pat Buchanan, a controversial politician and pundit who was recently sacked by MSNBC after critics deemed his new book antisemitic, racist and homophobic.