Cruz fires back at Trump's threat to 'spill the beans' on his wife
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Republican presidential candidate Ted CruzTed CruzSenate heading for late night ahead of ObamaCare repeal showdown Two GOP senators back ObamaCare repeal after Ryan call Four GOP senators threaten to block ‘skinny’ repeal MORE fired back at Donald TrumpDonald TrumpTrump cheers on Senate Republicans ahead of skinny ObamaCare repeal vote Live Coverage: Senate edges close to passing scaled-down ObamaCare repeal Pete King defends ‘unorthodox’ Scaramucci after attacks on Priebus MORE on Tuesday night after the front-runner said he would "spill the beans" on Cruz's wife.

"If you try to attack Heidi, you're more of a coward than I thought," Cruz wrote. 

Earlier in the night, Trump went after the Texas senator for an ad that used a photo of his wife, Melania, posing nude. The ad, by super-PAC Make America Awesome, was meant to appeal to Mormon voters in Utah, which held its caucuses Tuesday. 

In his response, Cruz said, "Pic of your wife not from us." 

Liz Mair, who runs the super-PAC, went on a Twitter blitz after the exchange, pointing out that Cruz wasn't behind the ad and speculating what Trump could have been referencing. 

It's possible that Trump is talking about a reported incident in 2005, when Heidi Cruz went through a period of depression and was picked up by police on the side of the road, according to BuzzFeed.

“About a decade ago, when Mrs. Cruz returned from D.C. to Texas and faced a significant professional transition, she experienced a brief bout of depression,” Jason Miller, an adviser to Cruz, told BuzzFeed

Trump and Cruz faced off Tuesday night in contests in Arizona and Utah.