Republican presidential candidate Ted CruzTed CruzHow 'Big Pharma' stifles pharmaceutical innovation AIPAC must reach out to President Trump Under pressure, Dems hold back Gorsuch support MORE on Thursday angrily slammed rival Donald TrumpDonald TrumpFundamentals or euphoria? Both fueled post-election stock surge Freedom Caucus founder: GOP health plan did not meet campaign promises Former US envoy: No good military options against North Korea MORE for dragging Cruz's wife into the campaign with a series of attacks.

"It's not easy to tick me off. I don't get angry often. But you mess with my wife, you mess with my kids, that'll do it every time," Cruz said in Wisconsin.

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"Donald, you're a sniveling coward. Leave Heidi the hell alone."

Trump first attacked Heidi Cruz on Tuesday night with a tweet threatening to "spill the beans" about her. Trump was responding to an ad from an anti-Trump super-PAC that featured Trump's wife posing nude.

Trump intensified the attacks by sharing a tweet late Wednesday night with an image comparing his wife, Melania, a former model, to Cruz's wife and stating, "A picture is worth a thousand words."

Asked whether he would support Trump if he gets the nomination, Cruz avoided the question.

"I'm going to beat him for the nomination," he said. 

"Donald Trump will not be the nominee," he continued to repeat when pressed.