Anderson Cooper grills Donald Trump. CNN/screenshot CNN anchor Anderson Cooper mocked Donald Trump on Tuesday for using a childish argument to defend a tweet about the wife of one of his rivals in the GOP primary.

During a CNN town-hall event for Republican presidential candidates, Cooper asked Trump to explain the feud between himself and Sen. Ted Cruz of Texas.

Cooper specifically challenged Trump's decision to share an image comparing the appearance of his wife to Cruz's. The tweet used an unflattering photo of Heidi Cruz.

"I didn't start it," Donald Trump offered to Cooper.

This was an apparent reference to an anti-Trump super PAC's online advertisement showing a racy photo of Melania Trump, a former model, from a GQ magazine photo shoot. The PAC isn't affiliated with the Cruz campaign.

But Cooper clearly didn't buy Trump's argument.

"Sir, with all due respect, that's the argument of a 5-year-old," Cooper said.

"No, it's not," Trump shot back.

"The argument of a 5-year-old is, 'He started it.' Every parent knows a kid who said 'He started it,'" Cooper said.

"You would say that," Trump said, talking over Cooper. "That's the problem with our country."

Trump's tweet last week about Heidi Cruz sparked one of the fiercest back-and-forths between the two candidates to date. Ted Cruz called Trump a "sniveling coward" and argued that candidates' wives should be off limits. Trump previously threatened to "spill the beans" on Cruz's wife in response to the super-PAC ad.