Fifty former Republican national security advisers, intelligence chiefs and trade representatives have signed a letter saying, "None of us will vote for Donald Trump."
They have served every U.S. Republican president from Richard Nixon to George W. Bush, and include former CIA Director Michael Hayden, former Homeland Security Directors Michael Chertoff and Tom Ridge, former Trade Representative Carla Hills and a number of ex-deputy secretaries of state.
It is some of the bitterest commentary on the Trump campaign by members of his own party to date.
The strongly worded letter not only says Trump is unqualified to be president, but that he would be a dangerous commander-in-chief and "the most reckless president in American history."
"He lacks the character, values and experience ... he weakens U.S. moral authority ... he lacks self-control and acts impetuously. He cannot tolerate personal criticism. He has alarmed our closest allies with his erratic behavior."
They accuse Trump of displaying "alarming ignorance of the basic facts of contemporary international politics" and say he shows no willingness to learn.
"All of these are dangerous qualities in an individual who aspires to be president and commander-in-chief with command of the U.S. nuclear arsenal."
Trump was equally as harsh, calling the 50 part of the "failed Washington elite" — insiders who, along with Hillary Clinton, made "disastrous decisions to invade Iraq, allow Americans to die in Benghazi and ... allowed the rise of ISIS [Islamic State]."
But his focus Monday was on the economy, and he unveiled his latest plans in the struggling Midwestern city of Detroit, headquarters of the U.S. car industry.
Trump's plans include large tax cuts for individuals and corporations, suspending regulations he says stifle business, and reviving the controversial Keystone oil pipeline that would run from Canada to the Gulf of Mexico.
He also said he would rewrite the North American Free Trade Agreement and pull the United States out of the Trans-Pacific Partnership, which the Senate has not yet passed.
He calls both trade agreements bad deals that give all the advantages to all signatories except the U.S.
"We now begin a great national conversation about economic revival for America," Trump said. "It's a conversation about how to make America great again for everyone ... especially for those who have the very least."
In a rare moment at a Trump rally, the sharp-tongued candidate stood by quietly as police escorted protesters out of the hall, instead of his making wisecracks and encouraging the audience.
In St. Petersburg, Florida, Democrat Hillary Clinton assailed the Trump plan, saying he is trying to make "old tired ideas sound new."
She said Trump is repackaging "trickle-down economics," a term used for benefits for the wealthy and corporations with the view that they will help the middle and lower class by reinvesting in the economy and creating jobs.
Historians say trickle-down economics has never worked, and Clinton cited economists who believe Trump's tax breaks for the wealthy would bring back recession.
Clinton plans to release her economic plan later this week. She already has said she would create millions of jobs by focusing on rebuilding crumbling infrastructure and making the rich pay "their fair share" of taxes.
Also Monday, a former CIA official and Republican congressional aide announced plans to run for president as what he calls a conservative alternative to Trump.
"America deserves much better than either Donald Trump or Hillary Clinton," Evan McMullin said Monday.
He will struggle to get his name on the ballot in many states, including some so-called battleground states where he could siphon votes away from Trump.
It is unclear what party, if any, McMullin would represent.