Dozens of the Republican Party’s most experienced national security officials will not vote for GOP presidential nominee Donald Trump, they wrote in an open letter released Monday.

“We are convinced that [Trump] would be a dangerous President and would put at risk our country’s national security and well-being,” said the former officials, many of whom held top positions in the George W. Bush administration.

“Most fundamentally, Mr. Trump lacks the character, values, and experience to be President,” they added. “He weakens U.S. moral authority as the leader of the free world. He 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.”

Signers include some of the best known intelligence, defense and security experts of the past two decades: Michael V. Hayden, the former director of both the Central Intelligence Agency and the National Security Agency; Michael Chertoff and Tom Ridge, both of whom served as secretaries of Homeland Security during the Bush administration; Dov Zakheim, a former under secretary of defense; John D. Negroponte, a deputy secretary of state and a former director of national intelligence; Eric Edelman, a top national security adviser to former Vice President Dick Cheney; and Robert Zoellick, a former deputy secretary of state, United States trade rep and president of the World Bank.

Trump said in response to the letter on Fox Business Tuesday morning that the people who signed the letter were not a part of his campaign but “would have loved to have been involved.”

“Look where the country is now on national policy. Look what we are in defense. Look where we are. Look at the mess we are in. Whether it’s the Middle East or anyone else,” he said.

“And these were the people that have been there a long time. Washington establishment people that have been there for a long time. Look at the terrible job they’ve done. I hadn’t planned on using any of these people,” Trump added.

“They don’t feel relevant because of that and they form a group and they go out and try to get some publicity for themselves and they hope that somebody else other than Trump wins because that way they can get a job.”

However, the letter, which was first reported on by The New York Times, represents yet another blow to Trump’s ongoing effort to win over top Republicans. That job that has become significantly more difficult in recent weeks, as Trump has feuded with the family of a fallen soldier and threatened repeatedly to abandon NATO.

The missive also raises questions about who might agree to serve in a hypothetical Trump administration and offer the former reality TV star advice on national security issues.

Trump has repeatedly sought to distance himself from some of the most controversial policies of the Bush administration, such as the war in Iraq, which Trump claims he opposed in 2003. Even so, it’s safe to assume that Trump’s campaign would have welcomed support from top members of the national security apparatus.

In closing, the 50 officials wrote, “We are convinced that in the Oval Office, he would be the most reckless President in American history.”

Read the entire letter below.

This article has been updated to include Trump’s reaction to the open letter on Fox Business.

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims ― 1.6 billion members of an entire religion ― from entering the U.S.