WASHINGTON (Reuters) - More than 90 Republican foreign policy veterans have pledged to oppose Donald Trump, saying his proposals would undermine U.S. security, in the latest sign of fissures between the Republican presidential front-runner and the party establishment. “Mr. Trump’s own statements lead us to conclude that as president, he would use the authority of his office to act in ways that make America less safe, and which would diminish our standing in the world,” the signatories wrote in a open letter on Wednesday. “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,” said the letter, which was posted on a blog called War on the Rocks. 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 2001-2009 presidency. Billionaire businessman Trump won the largest number of states in this week’s Super Tuesday nominating contests, intensifying moves by the party’s establishment wing to derail his path to the nomination. On Thursday, Trump in an interview with ABC News flatly rejected the criticism in the letter and blasted the nation’s military leaders as ineffective. Bryan McGrath, a retired U.S. Navy officer and adviser to Republican Mitt Romney’s unsuccessful 2012 presidential campaign who helped organize the letter, said at least two people declined to sign because of concerns it would fuel Trump’s campaign theme of being an anti-Washington candidate opposed by the establishment. “This is really drawing a bright moral line and saying that if we’re going to keep our souls, we can’t cross it,” said    Eliot Cohen, who served as counselor to former Secretary of State Condoleezza Rice and helped spearhead the letter. Rice was asked to sign but chose not to, in keeping with her general practice of not signing on to such group letters, a spokeswoman for the Stanford University professor said. The list of signatures, which numbered 60 when the letter was released on Wednesday night, had grown to 94 by Thursday afternoon. The signatories did not include other several high-profile former officials such as Bush national security adviser Stephen Hadley or former Secretary of State Colin Powell. It was unknown if they were invited to sign. The letter rejects numerous Trump foreign policy statements, including his “hateful, anti-Muslim rhetoric,” 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 signatories said. “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 “through a realist lens” on global affairs.  Trump has alarmed some mainstream Republicans with vows to shred 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 and has said he supports the practice of waterboarding and other similar interrogation techniques. Max Boot, a foreign policy adviser to Romney’s 2012 campaign and supporter of the Iraq invasion, was among the letter’s signers and said he “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.” Kurt Volker, a permanent representative to NATO under Bush, said he did not sign the letter because of concerns that any letter from “national security intelligentsia” could backfire. “He would actually use it as a bragging right,” said Volker, adding he had no intention of working for Trump but wanted to be free to offer advice to any future president, and that such a letter could prompt Trump to hold a grudge. Several others who declined to sign and asked not to be identified, said they feared it could help Democrat Hillary Clinton win the presidency. Trump told MSNBC on Thursday that he would name his foreign policy and national security advisers “in a week or two. ... But ultimately it’s my thought more than anybody else’s.” 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 response to the Sept. 11, 2001, attacks, has said he has been having regular talks with Trump but not in a formal role.  (Additional reporting by Phil Stewart, Arshad Mohammed and Susan Heavey; Editing by David Rohde, Stuart Grudgings, Bill Trott and Peter Cooney) This article was funded in part by SAP. It was independently created by the Reuters editorial staff. SAP had no editorial involvement in its creation or production.