Republican presidential candidate Donald Trump speaks about the results of Super Tuesday primary and caucus voting during a news conference in Palm Beach, Fla., on March 1, 2016. (Reuters/Scott Audette)

In a last-ditch effort to stop Donald Trump’s likely nomination as the Republican Party’s candidate for president, more than 50 conservative foreign policy experts have signed an open letter condemning the real estate magnate as unfit for the office.

From stating that he’ll make Japan — a close U.S. ally — pay for its longstanding American support, to vowing to kill the families of terrorists, Trump’s rhetoric appears to have finally crossed a line for those conservatives who have made their careers in foreign policy.

The letter’s signatories include former homeland security secretary Michael Chertoff, former deputy secretary of state Robert Zoellick, former homeland security adviser Frances Townsend and former undersecretary of defense Dov Zakheim — all of whom served under President George W. Bush. The letter was published Wednesday night on the foreign policy site War on the Rocks.

The letter stems from a Twitter back and forth by Eliot Cohen, a former State Department official in the Bush administration, and Bryan McGrath, a one-time naval adviser to former Massachusetts governor Mitt Romney’s 2012 presidential campaign.

“Do you want to do something about this?” McGrath recalls writing to Cohen, and soon they were bouncing a draft of the letter to their respective address books.

The short, bullet-pointed document hits Trump on issues including his praise for Russian President Vladimir Putin, his nonchalance about torture, being “fundamentally dishonest” and his anti-Muslim rhetoric.

According to Bloomberg View and a recent Reuters report, Trump is being “informally advised” by retired U.S. Army Lt. Gen. Michael Flynn. Flynn was chief of the Defense Intelligence Agency under President Obama from 2012 to 2014 and has long advocated for a warmer relationship with Russia.

“[Trump’s] vision of American influence and power in the world is wildly inconsistent and unmoored in principle. He swings from isolationism to military adventurism within the space of one sentence,” the letter reads.

The letter comes just days after Michael Hayden, the former head of the Central Intelligence Agency and the National Security Agency, said the U.S. military might disobey orders if Trump becomes president. 

McGrath said he has faced some criticism regarding the letter, notably that its signers are the exact type of establishment Republicans against whom Trump has been railing. He also said some have interpreted the letter as leaving open the possibility that instead of voting for Trump, McGrath and his co-signatories might instead support former secretary of state Hillary Clinton.

“I would never write a letter or sign a letter that carries that kind of implication,” McGrath said. “The only implication here is that we’re not going to support Donald Trump.”

McGrath added that he hopes voters see that the letter constitutes the views of “a reasonable group of people who work in this field” and that people realize Trump is unsuited for the job of president.

“I want to be on record saying that this man is not presidential material,” McGrath said. “And this was the best way I could do it and bring some friends along.”

This post has been updated