"Furthermore, his expansive view of how presidential power should be wielded against his detractors poses a distinct threat to civil liberty in the United States," it says.
The signatories include Robert Zoellick, a former World Bank president and deputy secretary of state; former U.S. Homeland Security Secretary Michael Chertoff; and Dov Zakheim, a top Pentagon official under President George W. Bush.
They represent both centrist Republican foreign policy circles and neoconservatives who favor a robust U.S. international role and wielded clout during Bush's 2000-2008 tenure.
Billionaire businessman Trump won the largest number of state nominating contests on Tuesday, intensifying moves by the party's establishment wing to derail his path to the nomination.
Bryan McGrath, a retired U.S. Navy officer and adviser to Mitt Romney’s unsuccessful 2012 presidential campaign who helped organize the effort, called the signatories "the right set of people". He said that at least two people declined to sign the letter, citing concerns it would only fuel Trump's campaign theme of being an anti-Washington candidate opposed by the establishment.
Eliot Cohen, who served as counselor to former Secretary of State Condoleezza Rice, also helped spearhead the letter, several people familiar with the effort said. Cohen would not comment.
Trump's campaign did not immediately respond to a request for comment.
The signatories did not include several high-profile former officials such as Rice, Bush national security advisor Stephen Hadley and former Secretary of State Colin Powell. It was not known if they had been asked to sign.
The letter, which was posted on a blog site called War on the Rocks, rejects numerous Trump foreign policy statements, including his anti-Muslim comments; his demand that Mexico fund a wall to control illegal immigration across the U.S. border; and his insistence that Japan pay much more for U.S. security assistance.
"As committed and loyal Republicans, we are unable to support a Party ticket with Mr. Trump at its head," the letter states. "We commit ourselves to working energetically to prevent the election of someone so utterly unfitted to the office."
The War on the Rocks blog calls itself a platform for former diplomats, military and intelligence officers and scholars to comment on global affairs "through a realist lens."
AMMUNITION FOR TRUMP?
Trump has alarmed mainstream Republican foreign policy and economic thinkers with comments vowing to tear up international trade deals. Many 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 Iraq invasion.
Boot is among the letter's signers, who also include David Shedd, who was acting director of the Pentagon's Defense Intelligence Agency.
Kurt Volker, who was a permanent representative to NATO under Bush, said he did not sign the letter out of concern it could end up backfiring.
"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.