In a blow to Donald Trump, a group of 50 Republican national security experts, including former top spy masters and diplomats, has said the White House nominee would be a dangerous President and would put the country's safety at risk.
However, Trump sought to dismiss their charges by saying that these "Washington elites" should look for answers on why the world has become a mess.
In a statement, the group of national security experts said, "None of us will vote for Donald Trump. From a foreign policy perspective, Trump is not qualified to be the President and Commander-in-Chief. Indeed, we are convinced that he would be a dangerous President and would put at risk our country's national security and well-being."
"Most fundamentally, Trump lacks the character, values, and experience to be President. He weakens the US' moral authority as the leader of the free world," said the letter which has been signed by individuals who served in senior national security and foreign policy positions in Republican administrations, from Richard Nixon to George W Bush.
The statement added that the 70-year-old business tycoon appears to lack basic knowledge about and belief in the US Constitution, US laws and US institutions, including religious tolerance, freedom of the press and an independent judiciary.
Claiming that Trump lacks the temperament to be the President, the statement alleged he has demonstrated repeatedly that he has little understanding of America's vital national interests, its complex diplomatic challenges, its indispensable alliances and the democratic values on which the US foreign policy must be based.
Trump, the statement said, is unable or unwilling to separate truth from falsehood. He does not encourage conflicting views.
"He lacks self-control and acts impetuously. He cannot tolerate personal criticism. He has alarmed our closest allies with his erratic behaviour. All of these are dangerous qualities in an individual who aspires to be President and Commander-in-Chief, with command of the US nuclear arsenal," they said.
The group joins members of the security establishment who have already come out against Trump, including former CIA director Michael Morell, who last Friday accused Trump of being an "unwitting agent" of Russian President Vladimir Putin.
Prominent among those who signed the letter were former CIA director Michael Hayden, former US trade representative Carla A Hills, former US Ambassador to India Robert Blackwill and former associate director for Near East affairs, National Security Council, White House Richard Fontaine.
Striking back, Trump, in a statement issued late Monday night, said, "We thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place. They are nothing more than the failed Washington elite looking to hold onto their power and it's time they are held accountable for their action."
Trump said the signatories, along with Democratic presidential nominee Hillary Clinton, are the owners of the disastrous decisions to invade Iraq, allow Americans to die in Benghazi, and they are the ones who allowed the rise of ISIS.
"Yet despite these failures, they think they are entitled to use their favour trading to land taxpayer-funded government contracts and speaking fees. It's time we put our foot down and declare that their gravy train is over: no longer will crooked Hillary Clinton and the other disasters in Washington get rich at our expense," he said.
Trump asserted that he has offered a better vision for the country and foreign policy -– one that is not run by a ruling family dynasty.
"It's an America first vision that stands up to foreign dictators instead of taking money from them, seeks peace over war, rebuilds our military, and makes other countries pay their fair share for their protection. Together, we will break up the rigged system in Washington, make America safe again, and we will make America great again," he said.
Top Republican Senator Collins joins anti-Trump bandwagon
Striking yet another blow, top Republican Senator Susan Collins has announced that she would not vote for Trump as he lacks the "temperament" and "judgement" required for the post, the latest party senator to renounce the presidential nominee.
"I will not be voting for Donald Trump for President," Collins, who represents the State of Maine in the US Senate, said in an op-ed in The Washington Post.
Trump, she argued, does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in the country.
Collins, the most senior senator to split publicly with the presidential nominee, said her conclusion about Trump's unsuitability for office is based on his "disregard" for the precept of treating others with respect, an idea that should transcend politics.
"Instead, he opts to mock the vulnerable and inflame prejudices by attacking ethnic and religious minorities. Three incidents in particular have led me to the inescapable conclusion that Trump lacks the temperament, self-discipline and judgement required to be President," she wrote.
Collins wrote, she is deeply concerned that Trump's lack of self-restraint and his barrage of ill-informed comments would make an already perilous world even more so.
"It is reckless for a presidential candidate to publicly raise doubts about honouring treaty commitments with our allies. Trump's tendency to lash out when challenged further escalates the possibility of disputes spinning dangerously out of control," she wrote.
Defending her decision not to support Trump, Senator Collins said some will say that as a Republican she has an obligation to support her party's nominee.
"I have thought long and hard about that, for being a Republican is part of what defines me as a person. I revere the history of my party, most particularly the value it has always placed on the worth and dignity of the individual, and I will continue to work across the country for Republican candidates," she said.
"It is because of Trump's inability and unwillingness to honour that legacy that I am unable to support his candidacy," Collins wrote.
She joined the few other Republican senators to repudiate the party’s nominee for president.
Senators Lindsey Graham, Mark Kirk, Ben Sasse, Dean Heller and Jeff Flake have all said they would either not vote for Trump or adamantly voiced their resistance to the notion.
Senator Ted Cruz, who was defeated by Trump in the primary race, had urged conservatives to "vote your conscience" at the Republican national convention.
Photograph: David Becker/Reuters