The speechwriter of former president George W Bush said he voted for Hillary Clinton as he struggled to support Republican nominee Donald Trump.
Conservative author David Frum wrote in an op-ed for The Atlantic that, although he had fiercely criticised Ms Clinton for years, he felt he "had to act decisively before it [was] too late".
"To vote for Trump as a protest against Clinton’s faults would be like amputating a leg because of a sliver in the toe; cutting one’s throat to lower one’s blood pressure," he wrote.
He added: "Trump is a man without political ideas. Trump's main interest has been and will continue to be self-enrichment by any means, no matter how crooked."
Mr Frum said he had "no illusions" about Ms Clinton, but that she was a "patriot".
"Your hand may hesitate to put a mark beside the name Hillary Clinton. You’re not doing it for her. The vote you cast is for the republic and the Constitution," he said.
His op-ed comes a day after the former president’s nephew, George P Bush, revealed at a rally in Texas that his uncle might "potentially" vote for the Democrat, joining his grandfather, former president George H W Bush.
In April, George W Bush’s wife, Laura Bush, hinted that she would like to see Ms Clinton as president due to her track record regarding women in Afghanistan. Ms Clinton wrote the forward for Ms Bush’s book on the subject in March.
Jeb Bush, the former president’s younger brother, ran against Mr Trump for the position of Republican nominee in the spring.
He said after he quit the race that he would not endorse his former opponent, unlike Ted Cruz and Marco Rubio.
