Donald Trump at a campaign event in Green Bay, Wisconsin. REUTERS/Eric Thayer Fifty former top Republican national security officials have signed an open letter in The New York Times opposing Donald Trump's candidacy, warning that he would be "the most reckless president in American history."

Many of the officials served in the administration of former President George W. Bush, but some served other Republican administrations.

"From a foreign policy perspective, Donald Trump is not qualified to be president and commander-in-chief," the letter said. "Indeed, we are convinced that he would be a dangerous president and would put at risk our country's national security and well-being."

The letter called into question Trump's temperament as well as his knowledge of US foreign policy issues.

"Most fundamentally, Mr. Trump lacks the character, values, and experience to be president," the letter said. "He weakens US moral authority as the leader of the free world. He appears to lack basic knowledge about and belief in the US Constitution, US laws, and US institutions, including religious tolerance, freedom of the press, and an independent judiciary."

It continued: "In addition, Mr. Trump has demonstrated repeatedly that he has little understanding of America's vital national security interests, its complex diplomatic challenges, its indispensable alliances, and the democratic values on which US foreign policy must be based."

The letter was signed by, among others, Michael Hayden, a former director of the CIA and the NSA; John Negroponte, a former director of national intelligence and deputy secretary of state; and Michael Chertoff, a former secretary of homeland security.

This isn't the first open letter from former national security officials who publicly oppose Trump, the Republican nominee for president. More than 100 Republican national security experts signed a similar letter in March.

The Times points out that some of the signatories on this letter refused to sign the first letter but changed their minds when they heard Trump jokingly invite Russian hackers to find Hillary Clinton's missing emails and suggested that he wouldn't help NATO allies in the event of a Russian attack if they didn't pay enough money for their own defense.

A significant number of national security officials have come out against Trump's candidacy, but he has built his campaign on an antiestablishment image, pushing the message that America's foreign policy is broken and the country needs to rely on fresh thinking from outsiders.