NORFOLK, Va. — With the polish of a seasoned politician, Khizr Khan strode through the door of a seafood restaurant to the serenade of clicking cameras, clasped hands with cheering Democratic lawmakers and, as he has become famous for doing, unflinchingly argued that Donald J. Trump must not be president.

To the naked eye, Mr. Khan, whose son was an American soldier killed in Iraq and who skewered Mr. Trump at the Democratic National Convention, could have been mistaken for someone running for office. But as he made his first appearance on the campaign trail for Hillary Clinton on Wednesday, he said his mission was greater than politics.

“Donald Trump as a candidate has proven himself to be temperamentally unfit to be the commander in chief of this great nation,” Mr. Khan said, his voice shaking. “There are no second chances.”

Mr. Khan emerged as one of the Clinton campaign’s most effective weapons after he made an impassioned plea for tolerance at the convention in July and accused Mr. Trump of not understanding the Constitution or personal sacrifice. When Mr. Trump responded by insinuating that Mr. Khan’s wife, Ghazala, had not spoken at the convention because she was muzzled by her Muslim faith, he faced intense backlash for attacking the grieving mother of a fallen soldier.