Former US vice president Joe Biden drew flak for calling a young woman a "lying, dog-faced, pony soldier" while campaigning in New Hampshire.
College student Madison Moore, 21, asked the presidential hopeful, 77, to explain a poor showing in the Iowa caucus last week.
"It's a good question," replied Biden, whose status as frontrunner for the Democratic presidential nomination was shaken by a fourth place in Iowa.
"Have you ever been to a caucus?" he then asked the Mercer University student, who nodded. "No, you haven't," Biden responded.
"You're a lying, dog-faced pony soldier."
Biden press secretary Remi Yamamoto said that was a joke, explaining: "It's from a John Wayne movie, and he's made it plenty of times."
But there were few laughs online. "If he can't handle a simple question from one of his own supporters how can Joe Biden take on Donald Trump?" asked Donald Trump Jr, the president's son.