DES MOINES — (CNN) Ted Cruz’s six-day bus trip in Iowa launched as more of religious revival tour than a presidential barnstorm. In the most conservative parts of the state on Monday, the Texas senator weaved in faith at every turn, soliciting shouts of “Amen” and prayers from the audience.

In Carroll, he asked the audience to “pray and ask God to continue this awakening.”

At a Christian bookstore in Boone, he quoted from the Book of Joshua.

“I was seeing Joshua 24:15 on the wall: ‘choose you this day whom you will serve; as for me and my house, we will serve the Lord,'” Cruz said. “And what a powerful reminder of the values that Washington, D.C., seems to have forgotten. What a powerful reminder of the values that built this country.”

But despite his outward displays of his belief, Cruz, the favorite in Iowa, is facing a whisper campaign of sorts from his Republican opponents: He’s Christian, but not Christian enough.

Donald Trump has cryptically questioned whether Cuba raises true evangelicals, pondering aloud whether Cruz is actually a Catholic. Ben Carson’s closest aide has argued that Cruz’s faith isn’t as native to him as Carson’s is to him. And a pair of back-of-the-pack contenders, Mike Huckabee and Rick Santorum, charge that Cruz is prioritizing wrong-headed Constitutional commitments over long-held Christian ones.

The attacks on the new Iowa frontrunner are just beginning. Their dark religious overtone is not subtle — and they could define the final four-week push to dethrone him from the top of the evangelical heap.

Cruz, who attends a Southern Baptist church and paces a stage like one of its preachers, has wooed the leaders of the evangelical grassroots for eight months, a courtship that is paying off this winter as he wins a string of high-profile endorsements. These growing victories also seem to have energized a batch of spurned rivals, some of whom have few cards left to play other than ripping the Iowa leader as a two-faced impostor with more political cunning than deep religious sincerity.

“There’s no reason to question his religious integrity,” said evangelical leader Rob Schenck, who deemed those blows “not helpful to the process.”

But Schenck acknowledged the politics at play: “Yes, it could definitely be damaging.”

The Christian rebuke is just one of many critiques leveled now at the GOP’s target du jour in Iowa. Santorum’s campaign on Monday became the sixth group in recent weeks to spend money against Cruz in Iowa with a new television advertisement. Other groups promise more to come.

Cruz, for his part, sees these attacks as a validation that he is a threat to win the nomination.

“You know, I’m looking at the materials some of y’all might’ve seen. ‘Ted Cruz is dangerous,'” he said, reading from a mailer on Monday afternoon given to attendees in Carroll by a pro-ethanol group. “You know what, to the Washington cartel, to the career politicians that are getting fat and happy and want the gravy train to keep going, that’s exactly right.”

And as he rides his bus throughout the state, he re-pitched himself to the state’s Christians, released a soft, direct-to-camera television ad in Iowa in which he promised to not “compromise the values that make us who we are.”

But on the ground, there are some signs that questions about Cruz’s commitment to the evangelical movement are breaking through. After his event in Carroll, Lea Hoffman approached Cruz and shared her concern about an audio tape (from a Cruz fundraiser in New York City) she had heard in an advertisement by Mike Huckabee’s super PAC which used it to portray Cruz as deemphasizing traditional marriage.

Hoffman told Cruz his son would vote for Rubio “because you are for gay rights.”

Cruz, ridiculing the “secret tape” as not-so-revealing, said he had a better record than Rubio on same-sex marriage. Hoffman later appeared convinced, telling CNN: “He lines up with what I believe the way the Lord wants this.”

Cruz’s consistent line has been that marriage is to be decided by the states. But that doubts are being stirred with like Hoffman is music to the ears of the Cruz rivals who charge that he is more corporate than Christian.

“If Senator Cruz is an advocate for states’ rights on marriage and on life, then I think would surprise a lot of evangelicals,” said Hogan Gidley, a spokesman for Huckabee, who supports constitutional amendments that would define marriage as being between a man and a woman.

“Ted Cruz is just like any other politician. He says one thing in Manhattan, he says another thing in Iowa,” Carly Fiorina said Sunday to CNN’s Dana Bash on “State of the Union.”

Other candidates have uncorked more biting remarks that speak to Cruz’s own personal life. After a period of peace between Cruz and Trump, the New York businessman last week once again implored Iowans to “just remember” that “not a lot of evangelicals come out of Cuba.” Trump suggested this weekend that it was possible Cruz, whose father was born there, is natively Catholic.

“Cuba, generally speaking, is a Catholic country. And you don’t equate evangelicals with Cuba. I don’t,” he said on CBS’s “Face the Nation.”

Even Ben Carson, who has been averse to much of the hand-to-hand combat that now defines the GOP race, has gotten involved: His business adviser and often-spokesman, Armstrong Williams, said on CNN on Thursday that Cruz, like Trump, was not an “authentic” evangelical Christian.

“There are a lot of people who talk a good game about their faith, including Mr. Trump and Mr. Cruz,” Williams said. “Dr. Carson lives it.”

Cruz allies say they are not worried about the attacks, no matter the realm. They maintain he has the political skill — and as of this weekend, the television time — to hold his lead in Iowa, even as he fights a battle against six outside groups and a dozen other GOP candidates. Allies placed $2 million in television advertisements in Iowa, thanks to a pair of super PACs — Keep the Promise and Stand for Truth — making their first big plays on television.

Details about the plans Stand for Truth, a mysterious new $4 million group formed independently from Keep the Promise but still blessed by Cruz’s big money network, were held close to the vest by group operatives on Monday.

But on the question of faith, the candidate’s defenders project confidence that his reputation will not be impugned.

“They don’t think he’s putting on a front just for an election,” said Tim Ried, an Iowa pastor supporting Cruz who is in touch with many other evangelical leaders in the state. “What he’s saying is what he is, and he’s lived that way for years before he started running.”

In Winterset on Monday evening, Cruz spoke of the emotion of his own family’s faith, especially given the rigors of the presidential campaign — revealing that he and his wife spent 30 minutes on the phone Sunday night in prayer with senior staff.

“Our prayer has never been, ‘We are seeking the hand of God,'” Cruz said of the prayer with his wife, Heidi. “We haven’t asked God give us your help in winning this race. That is not our prayer. We are not seeking his hand. Her prayer was, ‘Let us seek your face. Let this campaign reflect your love, reflect your glory and let your will be done.'”

And at the end of each of event, he had three asks for Iowa voters: to caucus for him, to sign up to volunteer and persuade others, and to pray and ask God to continue the awakening.

In Guthrie Center, he prayed: “I will hear their prayers and forgive their sins and I will heal their land; that is the promise we are standing on.”

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