A new ad released Thursday quotes Carter, who said Wednesday that if he had to pick between the two, he "would choose Trump" over Cruz.

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"Trump has proven already that he's completely malleable," Carter, a Democrat, continues in the clip. "I don't think he has any fixed opinions that he would really go to the White House and fight for. Ted Cruz is not malleable."

Carter was responding to a question at Britain's House of Lords on Wednesday about his assessment of American politics.

In his response, he explained that Cruz has far right-wing policies that he would pursue if he makes it to the White House.

The former president also said he had a feeling Trump's chances for the presidency would "fade away."

Cruz also used Carter's statements to his advantage at a New Hampshire town hall on Thursday, where he said that "Jimmy Carter endorsed Donald Trump," ABC News reported.

"I am not making that up. That is real," he said. "Jimmy Carter yesterday, he said, between Donald Trump and Ted Cruz, I would support Donald Trump. And he explained why, Carter said because Trump is malleable. He doesn't have any core beliefs, so he'll work with us and do what we want."

"But Cruz, that guy believes this stuff," he said.