50 wrote a lengthy letter, first published in the NY Times (reprinted here at Vox.com), explaining their reasons. Here are snippets.
The officials — who served in previous Republican administrations, including that of George W. Bush — recently signed a letter, published by the New York Times on Monday, essentially describing Trump as a national security threat. Here’s one particularly scathing section:
From a foreign policy perspective, Donald Trump is not qualified to be President and Commander-in-Chief. Indeed, we are convinced that he would be a dangerous President and would put at risk our country’s national security and well-being.
Most fundamentally, Mr. Trump lacks the character, values, and experience to be President. 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.
These claims aren’t exaggerations. In just the past month, Trump has defended his Muslim ban by attacking the family of a fallen veteran. He suggested that he would be open to essentially letting the US military alliance of NATO collapse. He told Republican Congress members that he’d defend a nonexistent article of the Constitution. And so on.
For national security officials, the possibility that someone who said all of these things could become president is a terrifying possibility — and, as they write, a threat to the US.
More: AZBlueMeanie covers this same letter and in addition reviews another letter by a former CIA operative with knowledge of Clinton's role in national security decisions (such as the Bin Laden raid). Snippets follow from that letter.
During a 33-year career at the Central Intelligence Agency, I served presidents of both parties — three Republicans and three Democrats. I was at President George W. Bush’s side when we were attacked on Sept. 11; as deputy director of the agency, I was with President Obama when we killed Osama bin Laden in 2011.
I am neither a registered Democrat nor a registered Republican. In my 40 years of voting, I have pulled the lever for candidates of both parties. As a government official, I have always been silent about my preference for president.
No longer. On Nov. 8, I will vote for Hillary Clinton. Between now and then, I will do everything I can to ensure that she is elected as our 45th president.
I spent four years working with Mrs. Clinton when she was secretary of state, most often in the White House Situation Room. In these critically important meetings, I found her to be prepared, detail-oriented, thoughtful, inquisitive and willing to change her mind if presented with a compelling argument.
In sharp contrast to Mrs. Clinton, Mr. Trump has no experience on national security. Even more important, the character traits he has exhibited during the primary season suggest he would be a poor, even dangerous, commander in chief.
To Russia with Love
President Vladimir V. Putin of Russia was a career intelligence officer, trained to identify vulnerabilities in an individual and to exploit them. That is exactly what he did early in the primaries. Mr. Putin played upon Mr. Trump’s vulnerabilities by complimenting him. He responded just as Mr. Putin had calculated.
Mr. Putin is a great leader, Mr. Trump says, ignoring that he has killed and jailed journalists and political opponents, has invaded two of his neighbors and is driving his economy to ruin. Mr. Trump has also taken policy positions consistent with Russian, not American, interests — endorsing Russian espionage against the United States, supporting Russia’s annexation of Crimea and giving a green light to a possible Russian invasion of the Baltic States.
In the intelligence business, we would say that Mr. Putin had recruited Mr. Trump as an unwitting agent of the Russian Federation.