Donald Trump's usual unflappable form deserted him at a rally in Dayton, Ohio. A video caught the moment Mr Trump became jumpy after an audience member behind him shouted. Security personnel immediately surrounded Mr Trump, who appeared to duck at hearing the noise.
The Republican presidential hopeful was then manhandled by security but did not leave the stage. The crowd is heard becoming increasingly raucous.
The show of nerves follows the cancellation of a rally in Chicago on Friday night after protests turned violent.
A campaign statement said: "Mr Trump just arrived in Chicago and after meeting with law enforcement has determined that for the safety of all of the tens of thousands of people that have gathered in and around the arena, tonight's rally will be postponed to another date."
"Thank you very much for your attendance and please go in peace."
Chicago police later said that they had not recommended that the rally was cancelled.
