WASHINGTON — John Negroponte, a career diplomat who served under three Republican administrations and as the first director of national intelligence, has endorsed Democratic presidential nominee Hillary Clinton.

Former US Air Force Secretary Michael Donley has, too, according the Clinton campaign. Donley was appointed to the position by President George W. Bush and also served under President Barack Obama. In his 38 years in the national security community, Donley said, it was the first time he was publicly endorsing a candidate.

"In this election cycle, there is no doubt that Secretary Clinton is the candidate best prepared to perform the president's duties as chief executive, head of state, and commander in chief," Donley said. "Her deep experience in public service, even temperament, willingness to listen to others and unifying message stands in stark contrast with Donald Trump, who has sown divisiveness at home, confused our allies abroad and shown repeatedly that he lacks the temperament, judgment, character and common decency the American people deserve and should expect in their leadership."

The news Wednesday comes days after a letter — signed by Negroponte and 49 other senior GOP national security officials — which rebukes the Republican presidential nominee as a "risk" to national security, citing Trump's "character, values and experience." The letter and the endorsement undermine Trump's efforts to win over establishment Republicans even as he runs as an outsider.

Among the other signatories are former CIA and National Security Agency Director Michael Hayden, as well as former secretaries of homeland security Tom Ridge and Michael Chertoff.

Though weeks earlier Trump and top Republicans had touted last month's GOP convention as cementing party unity, Trump's camp on Tuesday released a statement saying the signatories are the kinds of "insiders" he is running against. He also said they are to "blame for making the world such a dangerous place."

On Wednesday, Trump appeared on Fox News, firing back at Republican officials who oppose him, calling them "terrible people", "hangers-on", and "people that have given us a messed-up world."

"They're endorsing [Clinton] because I don't want them," Trump said. "The people that are there now, I don't want. Look at the world. Look at what's gone on with the Middle East. I mean, it's a mess. These are the people that are advising — these are the advisers?

"These are hangers-on. They couldn't make a living in the regular private sector, frankly," he said. "And they know I don't want them. I've never spoken to these people. I don't know who these people are."

Trump also said he would eliminate baseline budgeting and built-in increases for all government departments. He would, however, build up the military, he said.

"Our military is so depleted," he said. "We have to build up our military. There's nothing we can do about that. We've never needed a military just about so badly. So the military itself has to be built up. And we can't do anything about that."

"At the same time, I'm sure we can do it for a smaller price than most people would think because that's what I do. It's called negotiation," he said.

Republicans have seen a series of defections in recent days. The Clinton camp on Wednesday released a list of Republicans and Independents supporting her, which includes former Connecticut Rep. Chris Shays, former Commerce Secretary Carlos Gutierrez and former Treasury Secretary Hank Paulson.

Negroponte served as deputy national security adviser to President Ronald Reagan, as a diplomat under President Bill Clinton, and as director of national intelligence and deputy secretary of state for Bush. Negroponte also served as Bush's ambassador to the United Nations and then ambassador to Iraq.

Clinton became secretary of state under the succeeding Obama administration.

"I have personally known Hillary Clinton as First Lady, Senator and Secretary of State," Negroponte said in a statement. "She will bring to the Presidency the skill, experience and wisdom that is needed in a President and Commander-in-Chief. Having myself served in numerous diplomatic and national security positions starting in 1960, I am convinced that Secretary Clinton has the leadership qualities that far and away qualify her best to be our next President."

Donley said: "I don't agree with Secretary Clinton on every issue, but I am certain she fully understands what it takes to be president and that she has the knowledge and experience, leadership skills, and personal grit and dedication necessary to succeed in this most difficult task."

Twitter: @reporterjoe