Former secretary of state Colin Powell on Sunday endorsed Democratic presidential candidate Joe Biden, joining a growing chorus of Republicans and military leaders criticising President Donald Trump amid nationwide protests.
Mr Powell, a Republican who led the US military during the 1991 Gulf War in Iraq under President George HW Bush and later led the State Department under President George W Bush, said Mr Trump "lies all the time," has "drifted away" from the US constitution and poses a danger to American democracy.
"I cannot in any way support President Trump this year," Mr Powell, who did not vote for the Republican president in 2016, told CNN.
Mr Trump responded by calling Mr Powell a "real stiff" on Twitter.
The criticism comes as the nation faces widespread protests over police violence against black men and women, the coronavirus pandemic and a sharp economic downturn.
It is rare for Republicans to criticise Mr Trump directly, and more so for members of the military establishment, who typically stay out of politics.
Mr Trump's former defence secretary, retired general Jim Mattis, last week denounced what he called the president's deliberate efforts to divide the country.
Former Joint Chiefs of Staff chairmen Michael Mullen and Martin Dempsey have also criticised Mr Trump's handling of the unrest.
Republican senator Lisa Murkowski last week said she was "struggling" with whether to support Mr Trump's re-election, while Republican senator Mitt Romney praised Gen Mattis's words.
The New York Times reported on Saturday that George W Bush would not vote for Trump, quoting sources close to the only living Republican former president.
Many of the Republicans now criticising Mr Trump did not vote for him in 2016.
"President Trump has record support within the Republican Party," said Trump campaign spokesman Tim Murtaugh.
"However, the adoration of the liberal Beltway media is alluring and powerfully attractive to some DC insiders."
Since winning the White House, Mr Trump has secured an iron grip on Republican loyalty.
Many former critics, such as senator Lindsey Graham, have become ardent backers.
Republicans concerned ahead of election
Several Republican groups have formed to oppose Mr Trump's re-election, including a super political action committee, the Lincoln Project.
Then there is a new initiative called Republican Voters Against Trump, which last week launched a $10 million (Dh36.7m) digital advertising campaign.
"There are legions of privately concerned Republicans in Washington," said Tim Miller, who advised Republican Jeb Bush's 2016 presidential campaign and helped to launch Republicans Voting Against Trump.
"That doesn't do a whole lot for me. If you're not willing to say it, you may as well put on the red hat."
Mr Miller was referring to Mr Trump's signature campaign baseball cap, featuring the slogan "Make America great again".
Mr Powell said he would vote for Mr Biden, going a step further than some other Republicans who have said they do not support Mr Trump but have not backed his Democratic rival.
"I haven't voted for him so I'm not going to start now, but that's a different question from actively supporting Biden," said Mark Sanford, the Republican former South Carolina governor who mounted a brief primary challenge to Mr Trump last year.
The public denunciations from figures including Mr Powell and Ms Murkowski could sway more Republicans, Mr Sanford said.
"When these dominoes start to fall, they can fall a lot faster than people think," he said.