Paule Pachter, a Long Island Association board member, said that Mr. Powell was blunt.

“He said he would support Hillary Clinton and he also elaborated on several reason why he felt that Donald Trump was not the right candidate,” he said. “He spoke about his inexperience, he spoke about the messages that he’s sending out every day to his supporters, which really paints our country in a negative light across the globe with all our allies.”

The comments were a change from Mr. Powell’s tone in hacked emails from his inbox that were made public in September. In the emails, Mr. Powell criticized Mr. Trump but also expressed bitterness at Mrs. Clinton for repeatedly pointing to Mr. Powell’s email habits to explain away her own use of a private email server while she ran the State Department.

Mr. Powell’s comments make him the latest member of the Republican foreign policy and national security establishment to openly condemn his party’s nominee, and to say that Mrs. Clinton is the preferred choice.

But Mrs. Clinton came under harsh attack on Tuesday from Rudolph W. Giuliani, the former New York mayor, who echoed in more pointed language Mr. Trump’s recent assertions that Mrs. Clinton should be jailed.