Donald Trump. REUTERS/Mary Schwalm Real-estate magnate Donald Trump tweeted a challenge to Sen. Ted Cruz of Texas on Friday, encouraging his Republican presidential rival to attack him.

"Looks like @tedcruz is getting ready to attack," Trump wrote on Twitter.

"I am leading by so much he must," the Republican front-runner predicted. "I hope so, he will fall like all others. Will be easy!"

Trump was responding to a report in The New York Times the day before that quoted Cruz suggesting that voters would start to question the foreign-policy judgment of both Trump and another GOP candidate, retired neurosurgeon Ben Carson.

"You look at Paris, you look at San Bernardino, it's given a seriousness to this race, that people are looking for: Who is prepared to be a commander-in-chief? Who understands the threats we face?" Cruz reportedly told his supporters Wednesday at a closed-door meeting.

"Who am I comfortable having their finger on the button?" he continued. "Now that's a question of strength, but it's also a question of judgment. And I think that is a question that is a challenging question for both of them."

Cruz's campaign dismissed the story as "misleading," but Cruz himself said he would not comment on something he "may or may not have said at a private fundraiser." The Times responded by posting leaked audio of Cruz discussing Trump, which appeared to confirm the newspaper's account of what the senator had said.

The Times' report is notable as Cruz has had a striking camaraderie with Trump on the campaign trail. Cruz has been one of the few presidential candidates to refuse to criticize Trump amid Trump's multiple provocative comments and policy proposals.

"My approach — much to the frustration of the media — has been to bear-hug both of them and smother them with love," Cruz said of Trump and Carson in the leaked audio. "Look, people run as who they are. I believe that gravity will bring both of those campaigns down. And I think the lion's share of their supporters come to us."

In turn, Cruz had been the only notable Republican candidate whom Trump has refused to insult at his rallies. But that might be about to change.

In his Friday tweets, Trump wrote that Cruz shouldn't say things privately to his "bosses" — Trump's term for Cruz's donors — that he wouldn't say in public: