Anti-Trump wave courses through Republican ranks

More Republicans broke ranks Tuesday with their party's White House nominee Donald Trump, issuing dire warnings that his recklessness and lack of policy chops would put the United States in danger.

Facing sinking poll numbers and anger in the party over his repeated stumbles and divisive rhetoric, Trump's campaign has struggled to turn the page from a strong grassroots primary effort to a more mature head-to-head battle with Democrat Hillary Clinton.

On Monday, 50 former senior Republican national security officials warned in an open letter that if Trump were elected, he would be "the most reckless president in American history."

Republican presidential candidate Donald Trump, pictured on August 6, 2016, has garnered disdain from a large swath of America's political, defense and security establishment for his unorthodox -- some say downright dangerous -- views ©Scott Eisen (Getty/AFP/File)

Those tough comments drew a sharp reply from Trump, who painted the group as "nothing more than the failed Washington elite looking to hold onto their power" and saying they should be "held accountable" for making the world less safe.

Influential US Senator Susan Collins then piled on, saying the nominee was "unworthy" of America's highest elective office and would not receive her support.

On Tuesday, the intra-party bleeding continued.

The latest Republicans to jump ship are a pair of former Environmental Protection Agency administrators who blasted Trump for having "shown a profound ignorance of science" and for dismissing climate change as a hoax.

William Ruckelshaus, who served under presidents Richard Nixon and Ronald Reagan, and William Reilly, who worked with George H.W. Bush, said backing away now from crucial progress on global climate initiatives "would set the world back decades."

"We Republicans should be shocked, outraged even, at the prospect that all this progress, this legacy will be repudiated and rolled back by Donald Trump," they said in a statement released by Clinton's campaign, which they now support.

- 'Cruel comments' -

In an op-ed piece for The Washington Post, Collins -- a moderate senator from Maine -- did not pull punches.

"Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country," Collins wrote.

"I have become increasingly dismayed by his constant stream of cruel comments and his inability to admit error or apologize," wrote Collins, who portrayed Trump as a candidate "who will slash and burn and trample anything and anyone he perceives as being in his way or an easy scapegoat."

Trump has garnered disdain from a large swath of America's political, defense and security establishment for his unorthodox -- some say downright dangerous -- views, such as his professed admiration for Russian President Vladimir Putin and his apparent readiness to scuttle America's central role in the NATO military alliance.

Trump has also raised concerns over his recent war of words with the parents of a Muslim American soldier killed in a suicide bombing in Iraq.

Further to the shock and dismay of many in America's political class, he has even questioned why the nation has bothered to develop nuclear weapons if it has no intention of putting them to use.

Retired four-star general Barry McCaffrey spoke out against Trump, declaring him unfit for the Oval Office and saying it was "remarkable how little he knows" about national security.

"Why he's so out of the loop on these issues is hard to imagine," McCaffrey told MSNBC.

- Unfit for office -

While the US security experts did not say they would vote for Clinton, they declared Trump unfit for office, echoing Clinton's criticism by saying that he "lacks the character, values, and experience to be president" and displays "alarming ignorance of basic facts" of international politics.

The 70-year-old New York real estate mogul also has shown no willingness to learn about national security threats, "acts impetuously" and lacks self-control, said the experts, including former homeland security chiefs Tom Ridge and Michael Chertoff and former CIA director Michael Hayden.

Trump rejected the group as the "insiders" who, "along with 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," Trump said, using an acronym for the Islamic State jihadist group.

"I offer a better vision for our country and our foreign policy -- one that is not run by a ruling family dynasty."

While several GOP lawmakers have openly rejected Trump, Scott Rigell became the first House Republican to announce he will vote for Libertarian Gary Johnson in November.

"I think our party has made a terrible mistake in elevating Donald Trump," Rigell told MSNBC Tuesday.

Trump "is so lacking in judgment and temperament and in character that I think he really represents a true risk to our country, and I can not in good conscience support him."

Supporters of Republican presidential candidate Donald Trump at a rally on August 6, 2016 in Windham, New Hampshire ©Scott Eisen (Getty/AFP)

Senator Susan Collins, pictured on July 29, 2016, said Donald Trump was "unworthy" of America's highest elective office and would not receive her support ©Astrid Riecken (Getty/AFP/File)

Retired four-star general Barry McCaffrey, pictured in 2009, said it was remarkable how little Donald Trump knows about national security ©Chip Somodevilla (Getty/AFP/File)