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.

Two strongly held beliefs have brought me to this decision. First, Mrs. Clinton is highly qualified to be commander in chief. I trust she will deliver on the most important duty of a president — keeping our nation safe. Second, Donald J. Trump is not only unqualified for the job, but he may well pose a threat to our national security.

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.

I also saw the secretary's commitment to our nation's security; her belief that America is an exceptional nation that must lead in the world for the country to remain secure and prosperous; her understanding that diplomacy can be effective only if the country is perceived as willing and able to use force if necessary; and, most important, her capacity to make the most difficult decision of all — whether to put young American women and men in harm's way.

Mrs. Clinton was an early advocate of the raid that brought Bin Laden to justice, in opposition to some of her most important colleagues on the National Security Council. During the early debates about how we should respond to the Syrian civil war, she was a strong proponent of a more aggressive approach, one that might have prevented the Islamic State from gaining a foothold in Syria.

I never saw her bring politics into the Situation Room. In fact, I saw the opposite. When some wanted to delay the Bin Laden raid by one day because the White House Correspondents Dinner might be disrupted, she said, "Screw the White House Correspondents Dinner."

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.

These traits include his obvious need for self-aggrandizement, his overreaction to perceived slights, his tendency to make decisions based on intuition, his refusal to change his views based on new information, his routine carelessness with the facts, his unwillingness to listen to others and his lack of respect for the rule of law.

The dangers that flow from Mr. Trump's character are not just risks that would emerge if he became president. It is already damaging our national security.

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.

Mr. Trump has also undermined security with his call for barring Muslims from entering the country. This position, which so clearly contradicts the foundational values of our nation, plays into the hands of the jihadist narrative that our fight against terrorism is a war between religions.

In fact, many Muslim Americans play critical roles in protecting our country, including the man, whom I cannot identify, who ran the C.I.A.'s Counterterrorism Center for nearly a decade and who I believe is most responsible for keeping America safe since the Sept. 11 attacks.

My training as an intelligence officer taught me to call it as I see it. This is what I did for the C.I.A. This is what I am doing now. Our nation will be much safer with Hillary Clinton as president.

Michael J. Morell was the acting director and deputy director of the Central Intelligence Agency from 2010 to 2013.

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