Former Pennsylvania Gov. Tom Ridge, the first secretary of homeland security, joined 49 other senior Republican national security officials Monday in signing a letter that declares Donald Trump would "put at risk our country's national security and well-being" if elected.

Trump "would be the most reckless president in American history," the officials said, because he lacks the character, values and experience to serve in the office.

The letter - which was first reported by The New York Times - questions Trump's knowledge of, and commitment to, the U.S. constitution. It also says the GOP nominee has little understanding of the importance of strategic alliances, or of the democratic values that should undergird foreign policy.

Like Ridge, most of those who signed on to the open letter served in the administration of President George W. Bush.

Among the most prominent signers are Michael V. Hayden, a former director of both the C.I.A. and the National Security Agency; John D. Negroponte, who served as the first director of national intelligence and then deputy secretary of state; and Robert B. Zoellick, another former deputy secretary of state, United States trade representative, and president of the World Bank.

Michael Chertoff, a former U.S. attorney for New Jersey who also served as homeland security secretary, signed the document, as did Eric S. Edelman, the national security adviser to former Vice President Dick Cheney.

"The names on this letter are the ones the American people should look to for answers on why the world is a mess," Trump said in a statement. "We thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place."

It was a stinging rebuke to the GOP nominee from much of his party's national security establishment, and came after Trump has questioned the usefulness of the NATO alliance and appeared to invite Russia to set its computer hackers on the task of recovering emails deleted by Democratic Hillary Clinton when she was secretary of state. (Trump has said that the latter was a joke.)

Trump has said he plans to change the party's orthodoxy on foreign policy and national security matters.

Ridge has long been anti-Trump, declaring in March that he would not vote for the eventual GOP nominee.

"Every four years since my 18th birthday, I have pulled the lever in support of the Republican nominee for President of the United States. That streak will end this November," Ridge wrote in an op-ed in U.S. News & World Report. "With a bumper sticker approach to policy, his bombastic tone reflects the traits of a bully, not an American president and statesman. If he cannot unite Republicans, how can he unite America? I simply cannot endorse him."