Cruz fires back at Trump's threat to 'spill the beans' on his wife

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Republican presidential candidate Ted CruzTed CruzTrump enters new debate frontier Pence offers Cruz 'heartfelt thanks' for Trump endorsement Cruz: Trump hasn't apologized for personal insults MORE fired back at Donald TrumpDonald TrumpPost piles on: ‘Beyond debate’ Trump is unfit for office Clinton, Netanyahu have ‘in-depth’ conversation about US-Israeli ties NYT lays out argument against Trump for president 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.