More than 50 Republican foreign policy veterans have signed a letter pledging to oppose Donald Trump and rejecting his proposals, according to one of the coordinators of the effort, in the latest sign of fissures between the Republican presidential front-runner and the party establishment.

The letter will be posted early Thursday morning on the "War on the Rocks" foreign policy blog, according to one of the organizers, Bryan McGrath, a retired U.S. Navy officer who also advised Mitt Romney's unsuccessful 2012 presidential campaign.

"We've got the right set of people," said McGrath, founding managing director of The FerryBridge Group LLC consultancy. "It's Republican foreign policy, defense, international types who, in signing the letter, are pledging not to support Donald Trump."

McGrath, who said 55 people had signed the letter so far, did not identify the signatories and declined to release the contents of the letter. Trump's campaign did not immediately respond to request for comment.

The War on the Rocks blog calls itself a platform for former diplomats, intelligence officers and scholars to comment on global affairs "through a realist lens". It was not clear who hosts or funds it.

Two people with knowledge of the letter said it pledged signatories to do all they could to prevent a Trump presidency, citing several of his proposals, including building a wall along the Mexican border, threatening to impose tariffs on China and supporting waterboarding, a harsh interrogation technique that critics say amounts to torture.

Dov Zakheim, who served as undersecretary of defense under President George W. Bush, and Peter Feaver, who worked on Bush's National Security Council staff, confirmed by email that they signed the letter.

The Financial Times reported on Wednesday that Robert Zoellick, the deputy secretary of state under Bush and former president of the World Bank, had signed. Zoellick could not immediately be reached for comment.

Trump has alarmed mainstream Republican foreign policy thinkers with comments denigrating Muslims and Mexican immigrants, and vowing to tear up international trade deals. Many of them fear a Trump presidency would severely strain ties with allies, and are concerned about his stated willingness to work more closely with authoritarian Russian President Vladimir Putin.

Trump also has criticized the Republican party for its backing of Bush's 2003 Iraq invasion.

"I would sooner work for (North Korean dictator) Kim Jong Un than for Donald Trump. I think Donald Trump is objectively more dangerous than Kim Jong Un and not as stable," said Max Boot, who was a foreign policy adviser to Mitt Romney's 2012 campaign and supported the 2003 invasion. He said he had signed the letter.

AMMUNITION FOR TRUMP?

Boot and two other people said the anti-Trump effort was also being organized by Eliot Cohen, a Johns Hopkins University professor who served as Secretary of State Condoleezza Rice's counselor in the George W. Bush administration.

Cohen declined immediate comment.

McGrath said two people, who he didn't identify, declined to sign the letter because of their "fear that Donald Trump would use this as some sort of ammunition."

Kurt Volker, who was a permanent representative to NATO under the administration of George W. Bush, said he declined to sign the letter on concerns it could end up backfiring. It was not clear if he was one of the two experts mentioned by McGrath.

"My concern is that it's not smart for the intelligentsia - the national security intelligentsia - to come out and bash Trump, the candidate, partly, he would use that as a tool, saying: 'Here's the establishment. More of the same. They're afraid of me. I can do better.' He would actually use it as a bragging right."

Volker said he had no intention of working for Trump. But he also cautioned he wanted to be free to offer his advice to any future president, and that such a letter could prompt Trump to hold a grudge against signatories.

Several others who declined to sign, and asked not to be identified, said they did so because they feared such an effort could help Democrat Hillary Clinton win the presidency.

Trump's campaign has yet to release a full list of his foreign policy and national security advisers.

Those Trump has spoken with on foreign policy include a retired U.S. general and intelligence official, Michael Flynn, who favors closer ties with Russia. Flynn has declined to comment on whether he is advising Trump.

Former New York Mayor Rudy Giuliani, who won popularity for his handling of the Sept. 11, 2001 attacks, has said he has been having regular talks with Trump, but not in a formal role.