A group of 75 retired career Foreign Service officers, including ambassadors and senior State Department officials under Republican and Democratic presidents over nearly a half-century, has signed an open letter calling Donald Trump “entirely unqualified to serve as President and Commander-in-Chief.”

The diplomats said “none of us” will vote for Trump. While they said not all of them agreed with every decision made by Hillary Clinton, they said they all supported her candidacy.

“Because the stakes in this election are so high,” the letter said, “this is the first time many of us have publicly endorsed a candidate for President.”

The letter is the latest in an unprecedented number of joint public statements signed by retired high-level government officials and military officers this election cycle. Most have focused on national security, and most have been against Trump.

The most prominent exception was a letter early this month signed by 88 retired generals and other military officials who endorsed Trump as a “long overdue course correction in our national security posture.”

Last spring, more than 100 Republican national security experts signed a petition, even before Trump won the GOP nomination, saying they would never work for a Trump administration. Last month, 50 more Republicans, including former top aides and Cabinet members for the George W. Bush administration, signed a letter saying Trump would be “the most reckless president in American history” and that none of them would vote for him.

Most of the diplomats who signed the new letter have never been publicly associated with a political party. In their letter, they wrote that they “have proudly represented every President since Richard Nixon as ambassadors or senior State Department officials in Senate-confirmed positions. We have served Republican and Democratic Presidents with pride and enthusiasm.”

They had decided to speak out, the signers said, because “very simply, this election is different from any election we can recall.” Trump, they said, “is ignorant of the complex nature of the challenges facing our country, from Russia to China to ISIS to nuclear proliferation to refugees to drugs, but he has expressed no interest in being educated.”

The Trump campaign responded on Thursday that the country needs “an America First foreign policy.”

“How terribly weak and ineffective for a bunch of career overseas bureaucrats to send a letter or cable saying they want to keep things exactly as they are now and that they’re rallying around fellow insider Hillary Clinton,” the statement said. “The world has become a more dangerous place on their watch and they need to step up and own it.”

Although the idea for the letter was initiated by Nelson Cunningham, a former adviser to Democratic administrations, and James Keith, a former ambassador to Malaysia — both of whom now work for the international consulting firm McLarty Associates — many signers said they first saw it as it was widely distributed from friend to friend among the retired foreign policy ­community.

In interviews, several diplomats expressed broadly divergent reasons for signing. “As a normal issue,” former ambassador Ryan Crocker said, “generals or their civilian equivalent shouldn’t be making endorsements. I served six times as ambassador — three Republicans, three times for Democratic administrations — and I’m proud of that.”

“At the same time, looking at this campaign as I do through a national security optic, I am concerned enough to break my established position as nonpartisan,” said Crocker, whose ambassadorial posts included Afghanistan, Iraq and Syria. “I know Hillary Clinton a bit from my time in Afghanistan. I thought she was a terrific boss. She’s smart, focused, she knows how to make decisions.” The next president, he said, “is going to be dealing with a world of hurt, quite literally.”

“I don’t know Donald Trump,” Crocker said. “I don’t know what he’d do. But based on what he’s said, I know I don’t want to find out. This is scary.”

John Maisto, former ambassador to the Organization of American States, Venezuela and Nicaragua, who also served on Bush’s National Security Council staff, said the letter was “the first time I’ve ever done anything like that. I’m a registered independent.” His reasons, Maisto said, “are pretty straightforward — what the letter says.”

“The Republican candidate, as so many of my Republican friends have said, does not have the qualifications to do the job, across the board. In any way. And the Democratic candidate does have the qualifications. . . . She’s not perfect, but nobody is,” Maisto said.

Some said they signed for specific policy reasons. Edward Marks, the Ronald Reagan administration’s ambassador to Cape Verde and Guinea-Bissau, said, “I am very upset by the fact that Trump as a candidate has formally said he will use torture [and] . . . collective punishment as elements of U.S. policy. Those two pull him outside the normal U.S. political boundaries.”

Dan Kurtzer, former ambassador to Egypt and to Israel, said he objected to Republican support for a measure allowing U.S. citizens to sue foreign governments, which President Obama has said he would veto. He said he signed “not for the politics part, but literally for the protection that it [the measure] would strip away” from U.S. diplomats working overseas.”

Others have already publicly indicated a preference for Clinton, including Laura Kennedy, Bush’s ambassador to Turkmenistan, who has volunteered for the Clinton campaign. Nicolas Burns, undersecretary of state for Bush and a former ambassador to Greece, is rumored to be on a short list for Clinton’s secretary of state.

The letter’s signees also include Thomas Pickering, a veteran ambassador and senior diplomat who began his government work under President Harry Truman, and Marc Grossman, former ambassador to Turkey, who served as assistant secretary and undersecretary of state under Bush.

In statements about foreign countries, the diplomats wrote, “Mr. Trump has expressed the most ignorant stereotypes of those countries; has inflamed their people; and has insulted our allies and comforted our enemies.” And “shockingly, he has even offered praise and admiration for Vladimir Putin, the leader of Russia whose international activities and reported intrusions into our democratic political process have been among the most damaging actions taken by any foreign leader since World War II.”

By contrast, they said, “Hillary Clinton’s handling of foreign affairs has consistently sought to advance fundamental U.S. interests with a deep grounding in the work of the many tens of thousands of career officers on whom our national security depends.”