Are the two leading Republican presidential candidates—Donald Trump and Ted Cruz—truly Christian as they claim to be?

Doubts on the two candidates' true faith were raised separately on Tuesday by their fellow White House aspirants.

Talking to reporters while campaigning in Iowa, GOP presidential bet Jeb Bush called Trump a "flip-flopper" faith-wise, ABC News reported.

Bush was responding to a reporter's question if he thinks Trump is a Christian. The former Florida governor replied that he doesn't. "No, I don't know what he is," he said. "I don't think he has the kind of relationship he says he has if he can't explain it any way that shows he is serious about it."

Bush also commented on Trump's endorsement by evangelical leader and Liberty University President Jerry Falwell Jr.

"Trump views all this not from a position of his faith, he views it all as politics. I think faith goes way beyond politics," he said.

While Bush admitted that Falwell's endorsement was good for Trump, he questioned how this came about.

"For a guy who has stated he has never sought forgiveness, it's hard to imagine how someone who is a strong believer would embrace that idea. There may be other reasons that Mr. Falwell is supporting him," Bush said.

Liberty University alumni and others have also expressed their disappointment and "embarrassment" after Falwell formally endorsed Trump for president on Tuesday, Christian News reported.

Following Falwell's glowing remarks about Trump, some Christian leaders expressed sadness over his words.

"I cannot tell you how sad I am about the actions of Jerry Falwell, Jr. in his introduction of Donald Trump at Liberty University on Monday morning," wrote Michael Farris, chancellor of Patrick Henry College and founder of the Homeschool Legal Defense Association. "Not only did he compare Trump to his own father, he had the audacity to compare Trump to Jesus."

"Giving Trump an honorary doctorate in the past was unwise, but comparing him to Jesus was as close to heresy as I ever wish to witness," he said.

"Shame on you, Jerry Falwell, Junior for elevating success in business over the principles of right and wrong that flow from giving priority to the word of God over the priority of a balance sheet. I am deeply saddened."

Meanwhile, Trump's closest rival, Cruz, has likewise been labelled as a "phony" Christian.

Less than a week before the Iowa caucuses, supporters of GOP presidential bet Mike Huckabee launched a TV ad casting Cruz as a charlatan feigning intense Christian devotion to win votes, BuzzFeed reported.

The ad depicts two women chatting prior to a group Bible study about the upcoming Iowa caucuses. The women are shown discussing recent leaked comments in which Cruz privately told donors he wouldn't prioritise fighting same-sex marriage if he was elected; they also talk about Cruz's reportedly meagre record of charitable giving.

"He doesn't tithe?" one woman says in the ad. "A millionaire that brags about his faith all the time?"

"Just what we need — another phony," the second woman responds.

The ad ends with the first woman concluding, "Guess we've narrowed down our list. Can't caucus for Cruz," while her friends nods solemnly in agreement.

Nick Ryan, a Huckabee supporter, said the new ad seeks to expose Cruz as unworthy of the evangelical vote.

Ryan said he believed evangelical voters would find it especially revealing that Cruz donated less than 1 percent of his income to charity between 2006 and 2010, according to tax returns he released.

According to BuzzFeed News, some evangelicals are already questioning Cruz's lack of charitable giving.