A group of about 50 Republicans who have served in senior national security and foreign affairs positions in the White House have written a letter encouraging Americans not to vote for Donald Trump.

“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 states. “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,” the letter goes on. “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.”

“Many have doubts about Hillary Clinton…” the letter writers add. “But Donald Trump is not the answer to America’s daunting challenges and to this crucial election.”

“We are convinced that in the Oval Office, he would be the most reckless President in American history,” the letter concludes.

Aaron Friedberg, who served as Deputy Assistant for National Security Affairs under Vice President Dick Cheney, told CTV News Channel that he signed the letter hoping to convince undecided voters.

“We hope to encourage others who have perhaps not made up their minds to look very carefully at what he’s saying and whether he’s the person they want as their Commander in Chief,” said Friedberg, who is a professor at Princeton University.

He added that he believes voters are “frustrated with government (and) don’t like many for the things that have been going on in the country for the last eight years,” but that Trump is simply “unsuitable.”

Trump addressed the letter in a speech Tuesday, telling supporters that these “politicians” who are not supporting him “are real geniuses.”

“These are 50 people that have been running our country forever and they said ‘we can’t support Donald Trump,’” Trump said. “The reason they can’t, you know why?” he added. “Because I’m not going to hire these people; I don’t want these people.”

Among the highest-profile Republicans who signed are former CIA director Michael Hayden, and Michael Chertoff, who helped write the Patriot Act anti-terror law as Secretary of Homeland Security under George W. Bush.