Inside a sleek Denver condominium, George W. Bush let a hundred donors to his brother’s campaign in on a secret. Of all the rival Republican candidates, there is one who gets under the former president’s skin, whom he views as perhaps Jeb Bush’s most serious rival for the party’s nomination.

It isn’t Donald Trump, whose withering insults have sought to make Jeb pay a political price for his brother’s presidency. It isn’t Marco Rubio, Jeb’s former understudy who now poses a serious threat to his establishment support.

It’s George W. Bush’s former employee — Ted Cruz.

“I just don’t like the guy,” Bush said Sunday night, according to conversations with more than half a dozen donors who attended the event.

One donor in the room said the former president had been offering mostly anodyne accounts of how the Bush family network views the current campaign and charming off-the-cuff jokes, until he launched into Cruz.

“I was like, ‘Holy sh-t, did he just say that?’” the donor said. “I remember looking around and seeing that other people were also looking around surprised.”

“The tenor of what he said about the other candidates was really pretty pleasant,” another donor said. “Until he got to Cruz.”

Bush took a harsh view of Cruz’s apparent alliance with Trump, who stood with the senator at a Capitol Hill rally last month in opposition to the Iran deal. While Trump, the current GOP poll leaders, has attacked most of his competitors in the 2016 field, he has avoided criticizing Cruz.

One donor, paraphrasing the former president’s comment in response to a broad question about how he viewed the primary race and the other Republican candidates, said: “He said he found it ‘opportunistic’ that Cruz was sucking up to Trump and just expecting all of his support to come to him in the end,” that donor added.

George W. Bush is well acquainted with his home-state senator, who served as a domestic policy adviser on his 2000 campaign before rising to national prominence by distancing himself from — and often going out of his way to antagonize — the GOP establishment. In his book published earlier this year, Cruz ripped Bush’s record, criticizing elements of his foreign policy and faulting the administration for enabling "bigger government and excessive spending and new entitlements."

While Jeb Bush’s campaign is spending far more time of late pushing out information that contrasts favorably with Rubio, his oldest brother seemed to see Cruz as the biggest threat in the end. According to several donors, the former president said not to doubt Cruz’s strength.

“He said he thought Cruz was going to be a pretty formidable candidate against Jeb, especially in Texas and across the South,” a donor said.

A spokesman for the former president pushed back at the takeaway that he views Cruz as his brother's main obstacle in the 15-candidate primary field.

"The first words out of President Bush's mouth last night were that Jeb is going to earn the nomination, win the election, and be a great president," said Freddy Ford, George W. Bush's spokesman. "He does not view Sen. Cruz as Gov. Bush's most serious rival."

Cruz's campaign, after initially declining to comment for this story, provided a statement from the senator Monday night.

“I have great respect for George W Bush, and was proud to work on his 2000 campaign and in his administration," Cruz said. "It's no surprise that President Bush is supporting his brother and attacking the candidates he believes pose a threat to his campaign. I have no intention of reciprocating. I met my wife Heidi working on his campaign, and so I will always be grateful to him."

The donors at the event were a mix of establishment stalwarts like former Gov. Bill Owens and business executive Larry Mizel as well as a number of young professionals, who were offered reduced $250 tickets at the last minute in an effort to fill the room, according to an email the organizer circulated among potential supporters and obtained by POLITICO.

The former president was softer in critiquing Rubio, whom many view as the biggest threat for establishment support and, by extension, the nomination itself. After recognizing Rubio’s political and rhetorical skills, George W. Bush reprised the common criticism of the first-term senator — experience. But then, jokingly, he undercut himself.

“He’s a young, first-term senator; I’m not sure if that qualifies you to be president,” Bush reportedly said, according to two people in the room. “Of course, if he wins [the nomination], I’ll be back here next year telling you that doesn’t matter.”

Bush also cast Cruz’s candidacy as an exercise in personal gain, not service. “He sort of looks at this like Cruz is doing it all for his own personal gain, and that’s juxtaposed against a family that’s been all about public service and doing it for the right reasons," a donor said. "He's frustrated to have watched Cruz basically hijack the Republican Party of Texas and the Republican Party in Washington."

The former president, who lives in Dallas, was appearing at his fourth fundraiser in less than a month for his brother’s campaign, which is hoping to capitalize on the family legacy and donor network without allowing the 43rd president’s controversial record to become a distraction. He will also be appearing along with his father, former President George H.W. Bush, at an event in Houston this weekend that was set up to reward new donors to Jeb Bush’s campaign with the rare opportunity to see two former presidents and, potentially, a third in the same room.

During some 35 minutes of unscripted remarks inside billionaire industrialist Lanny Martin’s condominium adjacent to the Daniel Libeskind-designed Denver Art Museum, Bush never addressed his own record, even as his brother was defending it in another war of words with Trump. The developer and entertainer blamed the former president in part for the 9/11 terrorist attacks because they happened on his watch and for the Iraq War.

Instead, George W. Bush offered the same “long haul” view of the race espoused by his brother’s Miami-based campaign and sprinkled in several colorful family anecdotes. He spoke movingly about Jeb’s courtship of his eventual wife, Columba, whom he met when he was just 16 and a student in Mexico.

“Let’s just say it was a surprise for our family,” Bush reportedly said of his brother’s wedding at the age of 21 to the former Columba Garnica de Gallo.

In touting his brother’s experience and qualifications to be president, Bush acknowledged their differences, emphasizing his younger brother’s thoughtful, deliberative nature. He asserted that Jeb’s more inclusive tone on immigration would make him the party’s best hope of drawing Hispanic voters back to the Republican Party. (George W. Bush was the last Republican to win a large percentage of the Latino vote.)

Bush gave further definition to the brothers’ contrasting personalities by showing off a self-deprecating sense of humor.

“He was talking about the need to reach out to immigrants and he said, ‘There’s a lot of people in this country who speak broken English,’” a donor recounted. “And then he paused and said, ‘Well, I’m one of them.’”

Despite his lasting appeal to a large swath of establishment Republicans, the former president told donors it’s unlikely he would hit the campaign trail for his brother at any point. Jeb Bush, asked in Iowa two weeks ago if he’d been grappling with how to best employ his oldest brother, was noncommittal but emphatic that “there’s no grappling going on.”

That seems to be the case.

“He emphasized that he’s not going to be out on the campaign trail doing public events,” one donor said. “He wants to be helpful and supportive like any brother would be. But he said, ‘You’re not going to see a lot of me.’

“Actually, he said that a couple of times.”