Brent Scowcroft, who was national security adviser to two Republican US presidents, has endorsed Democrat Hillary Clinton for president over Donald Trump.
"The presidency requires the judgment and the knowledge to make tough calls under pressure," the Republican elder statesman said in a statement.
"I believe Hillary Clinton has the wisdom and experience to lead our country at this critical time."
Scowcroft, 91, served as national security adviser under Presidents Gerald Ford and George HW Bush. Ford was president from 1974 to 1977 and Bush from 1989 to 1993.
He joined a number of well-known Republicans defecting due to the prospect of a Trump presidency, including Richard Armitage, deputy secretary of state under President George W Bush.
Armitage gave Clinton, 68, who was secretary of state under Democratic President Barack Obama, his backing last week. Trump "doesn't appear to be a Republican, he doesn't appear to want to learn about issues. So, I'm going to vote for Mrs Clinton," Armitage told Politico at the time.n.
Trump, 70, who has never held elected office, often boasts on the campaign trail his relative newness to politics, appealing to voters disillusioned by the current political climate.
