Former President George W. Bush slammed Texas Sen. Ted Cruz at a fundraising event for his brother Jeb Bush's presidential campaign on Sunday, reportedly saying he thought Cruz was "hijacking" the GOP but also acknowledging that Cruz was likely his brother's most formidable opponent.

"I just don't like the guy," Bush told donors to his brother's campaign in Denver, multiple anonymous donors told Politico.

Bush criticized Cruz for his de facto alliance with Republican front-runner Donald Trump, saying that Cruz appears to be engaged in political opportunism and "was sucking up to Trump and just expecting all of his support to come to him in the end," one donor told Politico.

"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," the donor said. "He's frustrated to have watched Cruz basically hijack the Republican Party of Texas and the Republican Party in Washington."

Trump has issued scathing attacks against practically every candidate in the 15-person field, but Cruz has been notably spared and has declined to speak out against Trump.

Cruz served as a domestic policy adviser on George W. Bush's 2000 presidential campaign and then as part of the recount team that helped Bush win the election. He went on to work under Bush as the Department of Justice coordinator for the Bush-Cheney transition team, as the associate deputy attorney general at the DOJ, and at the Federal Trade Commission, according to the National Review. Cruz then departed to become the solicitor general of Texas and rose to national prominence by framing himself as anti-establishment.

Bush also warned donors Sunday that Cruz was probably his brother's biggest threat and to not underestimate his strength, especially in the South and in Texas.

"He said he thought Cruz was going to be a pretty formidable candidate against Jeb," a donor said.

George W. Bush spokesman Freddy Ford denied the former president's remarks about Cruz, according to Breitbart. "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. He does not view Senator Cruz as Governor Bush's most serious rival," Ford said.

Cruz initially declined to comment, but his campaign eventually released a statement saying he has "great respect for George W. Bush, and was proud to work on his 2000 campaign and in his administration."

"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 statement said, according to Fox News.