A former national security aide to President George W. Bush says Republican presidential nominee Donald Trump has shown no interest in “learning the responsible behavior of a statesman” and warned that the GOP standard-bearer would risk the country’s national security and diplomatic relations abroad.

“A combination of Trump’s extremely irresponsible, provocative and frankly ignorant statements about a whole range of national security issues, coupled with his complete lack of any experience in this field whatsoever,” would make him the “most reckless president we’ve ever had,” William Inboden told The Texas Tribune in a Wednesday interview.

Inboden, an assistant professor of public affairs and executive director of the William P. Clements Jr. Center for National Security at the University of Texas at Austin, assisted Bush on foreign policy issues, including counterterrorism and diplomatic issues. In the interview, he said top GOP leaders should have denounced Trump’s controversial remarks at the time he launched his presidential bid.

“At this point, Trump has turned into his own worst enemy, so I don’t think any more denunciations are necessary,” he said.

Inboden’s comments came days after he and 49 other former senior Republican national security heavyweights signed a scathing letter saying that Trump “lacks the character, values and experience” to be president and “appears to lack basic knowledge about and belief in the U.S Constitution, U.S. laws and U.S. institutions, including religious tolerance, freedom of the press and an independent judiciary.”

Trump responded to the letter by calling the signatories part of the Washington establishment he’s fighting against, adding that “they are nothing more than the failed Washington elite looking to hold onto their power.”

In June 2015, when he launched his presidential bid, Trump claimed that Mexico was sending “rapists” and other criminals into the U.S. And during his campaign events, he often renews his vows to force the Mexican government to pay for a wall along the U.S.-Mexico border and to institute a temporary ban on Muslim immigrants entering the country. His harsh remarks about women, immigrants and Muslims have triggered nationwide concerns and denunciations, even among GOP stalwarts.

While House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell — the highest-ranking members of the Republican Party — have been trying to distance themselves from Trump’s campaign rhetoric, Inboden said if they alienate the real estate mogul and his supporters, the GOP would struggle in down-ballot races.

“Politically, it’s a very difficult position to be in,” he said.

The national security letter said many Americans have doubts about Democratic presidential nominee Hillary Clinton, “as do many of us.” But it suggested that Trump “is not the answer to America’s daunting challenges and to this crucial election.”

Inboden was quick to point out that he won’t be voting for Clinton in November; he’s not going to vote in the presidential election at all.

He said the fact that Trump responded to the national security letter “with name-calling and ignorance, I think it proves the letter’s point.”