President Gerald Ford, second Presidential Debate, October 20, 1976:
"There is no Soviet Domination of Eastern Europe, and there never will be under the Ford Administration."
Ford made matters even worse in the days following the comment by saying what he'd said was true. It was up to Ford's Chief of Staff, Dick Cheney to try to mitigate the comment by "clarifying" it at a tumultuous press event. Ford had been holding a 2-4 point lead over Jimmy Carter just prior to the second debate in 1976, but that lead evaporated into a 4-point loss in the general election two weeks later.
Senator John Kennedy, fourth Presidential Debate, October 17th, 1960:
"Anyone reading the papers and any citizen of the United States must come to the conclusion that the United States no longer commands the same image of a vital society on the move, with its brightest days ahead, as it carried a decade or two ago."
John Kennedy had been building a slight lead over Richard Nixon, who had lead throughout the campaign that early fall, until the first televised debate, which Nixon fumbled with his poor television appearance and optics. He'd recovered in the matter of an imminent attack by the Chinese Communists over the tiny Pacific island of Qumoy and Matsu, which Kennedy didn't answer well. In the fourth and final debate, Kennedy made an allusion to the diminution of American power and prestige, which many found offensive. Going into election day, Kennedy's tiny lead had evaporated. The result, as we all know, was razor-thin.
President Ronald Reagan, second Presidential Debate, October 19th, 1984:
"No, Mr. Trewitt, and I will tell you that I will not make age an issue of this campaign, and I will not exploit for political purposes the youth and inexperience of my opponent."
Reagan, by his own admission, lost the first debate against Walter Mondale, by "cramming my head with too many facts and figures". He'd thus appeared tired and old. Right off the bat, at the beginning of the second debate, Reagan go this zinger in-- and, with Walter Mondale standing at his side laughing heartily at the joke, sewed up an election that had tightened somewhat after the first debate. "After that response by Reagan", Mondale said years later "I knew I'd lost the election".
Governor Ronald Reagan, GOP Primary Candidates Debate, Nashua, New Hampshire, February 23rd, 1980:
"Is this on? You asked me... Mr. Green, I am paying for this microphone!"
The moderator at the event tried to silence Reagan's opening statement by having his microphone turned off. You don't do that to an old radio hand, and expect to live through it.
Some are already complaining that there are too many participants in the debates. Au Contraire. A quip here, a flub there...
Things can change. Quickly.