Story highlights Duck Dynasty's Phil Robertson said GOP presidential candidate Ted Cruz could restore the U.S. to biblical principles

Robertson argued that the country's moral core is collapsing

Iowa City, Iowa (CNN) As Donald Trump and Ted Cruz battled for evangelical voters across Iowa Sunday, Cruz's star surrogate, Duck Dynasty's Phil Robertson, made a forceful case to voters that the Texas senator could restore the U.S. to biblical principles and called gay marriage "evil" and "wicked."

Warming up a large crowd for Cruz in Iowa City as Trump campaigned with evangelical leader Jerry Falwell Jr. in Council Bluffs, Robertson argued that the country's moral core is collapsing.

The tirade against gay marriage played well in the room, but would likely raise red flags for voters who are weighing whether Cruz is the kind of GOP nominee who could attract swing voters. Robertson said Cruz was the only candidate who could restore the constitutional and biblical foundation of government.

"When a fellow like me looks at the landscape and sees the depravity, the perversion -- redefining marriage and telling us that marriage is not between a man and a woman? Come on Iowa!" Robertson said.

"It is nonsense. It is evil. It's wicked. It's sinful," he said to applause. "They want us to swallow it, you say. We have to run this bunch out of Washington, D.C. We have to rid the earth of them. Get them out of there."

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