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Now, the real estate magnate and current Republican presidential front-runner is preparing to set his sights on the Texas senator.
Trump has torn through various rivals already -- repeatedly calling former Gov. Jeb Bush "low energy," raising questions about Ben Carson's "pathological" temper, and so on -- but had previously been fairly friendly with Cruz.
The pair appeared at the same event in Washington in September to slam the Obama administration's Iran nuclear deal, spreading mutual admiration and sharing the spotlight at the rally, but it appears that times may be changing.
At a closed-door donor event, Cruz is heard questioning if either Trump or Carson have the proper judgment to become president, as heard on an audio recording provided by The New York Times. For the full recording go to nytimes.com.
"Who am I comfortable having their finger on the button? Now, that's a question of strength, but that's also a question of judgment and I think that is a challenging question for both of them," Cruz said.
He also is heard explaining the strategy that has helped him avoid openly criticizing either in the past, and effectively removing the target from his back.
"So, my approach, much to the frustration of the media, has been to bear hug both of them and smother them with love," Cruz is heard saying on the audio recording.
The Cruz campaign has called The New York Times report misleading and in an appearance on Fox News on Thursday, Cruz insisted his comments were not specifically about Trump, despite clearly referencing the candidate in the audio.
"What I have said many times is particularly in the wake of the terrorist attack in San Bernardino, that the American people are looking with greater seriousness and gravity at a commander-in-chief,” Cruz said. “They are making a judgment of every candidate who has the experience.”
Trump did not take kindly to the slight.
.@tedcruz should not make statements behind closed doors to his bosses, he should bring them out into the open - more fun that way!— Donald J. Trump (@realDonaldTrump) December 11, 2015
Trump also suggested on Twitter that Cruz will "fall like all others.”
Looks like @tedcruz is getting ready to attack. I am leading by so much he must. I hope so, he will fall like all others. Will be easy!— Donald J. Trump (@realDonaldTrump) December 11, 2015
Earlier this week, a Monmouth University poll of Iowa voters showed Cruz slightly ahead of Trump, though another poll released later the same day put Trump ahead. Cruz was cautious, saying that despite his lead over the businessman, he has no intention of going after the billionaire real estate mogul, whom he has been careful not to criticize -- even during a week when Trump’s controversial plan to bar Muslims from entering the U.S. has been the talk of the campaign trail.
"I like and respect Donald Trump, I continue to like and respect Donald Trump,” Cruz told reporters while campaigning in South Carolina earlier this week. "While other candidates in this race have gone out of their way to throw rocks at him, to insult him, I have consistently declined to do so and I have no intention of changing that now."
This summer, the two men held a nearly hour-long meeting after which they vowed not to let the fact that they are campaigning for the same office get in the way of their friendship.
"He's running and I'm running and we both agreed that we started out as friends and we'll end as friends," Cruz said in July.
After the rally in September, Cruz praised Trump and acknowledged his rival's ability to draw a crowd.
"Number one, I like Donald, but number two, everywhere Donald goes, he brings 100 television cameras with him," Cruz said.
But the signs that the bonhomie between the two men may be at risk have been increasing lately. At a campaign rally in South Carolina on Monday, Trump “promised” that he would go after Cruz if the Texas senator attacked him.
"I'll tell you something, in the end, the people that give these millions of dollars to these horrible corrupt PACs, they're corrupt. They totally control Bush, Rubio. I won't say Cruz because he's been nice to me. He's got to hit me first. Once he hits me, I promise you. I promise you," Trump said.
For his part, Cruz appears to be trying to head off a fight.
Not to be outdone, Trump went on to re-tweet Cruz's message.