Warning comes as former CIA counter-terrorism officer announces he will run for president
Fifty Republican national security experts have signed an open letter warning that Donald Trump "would be the most reckless president" in US history.
The group, which includes the former CIA director Michael Hayden, says Mr Trump "lacks the character, values and experience" to lead the country.
The letter states: "None of us will vote for Donald Trump.
"From a foreign policy perspective, Donald Trump is not qualified to be 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."
It comes as a little-known former CIA counter-terrorism officer announced he is to run for president against Mr Trump.
Evan McMullin is reportedly being put forward as a third-party conservative alternative by Republicans disillusioned with their party's candidate for November's White House election.
Like Mr Trump, the 40-year-old Mormon has never held elected office, but unlike Mr Trump he is completely unknown to American voters.
Mr McMullin most recently served as chief policy director for the US House of Representatives' Republican Conference.
He has worked in Congress since 2013, according to a LinkedIn profile.
The House Republican Conference said in a statement he is no longer employed there.
Mr McMullin previously spent 11 years as an operations officer for the Central Intelligence Agency, according to reports.
He said in a statement on his campaign Facebook page: "It's never too late to do the right thing, and America deserves much better than either Donald Trump or Hillary Clinton can offer us.
"I humbly offer myself as a leader who can give millions of disaffected Americans a better choice for President.
Mr McMullin had been a fierce critic of Mr Trump on social media, saying that opposing the businessman is about "putting principle over power".
Authoritarians like @realDonaldTrump use promises of law & order to justify infringing on civil rights as they consolidate control by force.— Evan McMullin (@Evan_McMullin) July 22, 2016
His candidacy underlines how the Never Trump diehards are still trying to derail the campaign of their own party's standard-bearer, barely three months before voting day.
Mr McMullin is seen as having zero chance of succeeding President Barack Obama in the White House.
He has already missed the deadline to get on the ballot in Texas, North Carolina, Illinois and Florida.
It seems the very best he could hope for would be to peel away a few votes from Mr Trump in a handful of states.
But his campaign could also backfire, uniting Republicans against a third-party candidate who might ultimately serve to boost Democratic candidate Hillary Clinton.
Meanwhile, in a statement, Mr Trump hit back at those who signed what he called the "politically motivated" letter.
"The names on this letter are the ones the American people should look to for answers on why the world is a mess, and we thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place," he wrote.
"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 actions."
Earlier, at a speech in Detroit, Michigan, Mr Trump put some flesh on his economic plans.
He said no business should pay more than 15% of income in taxes.
Mr Trump also proposed allowing parents to fully deduct the average cost of childcare from their taxable income.
He proposed, too, cutting the top rate of federal income tax to 33% from 39.6%.
Mr Trump is planning to introduce a slew of new policy proposals in the coming weeks in an effort to steady a campaign that appears to have been floundering recently.