"mud slinging"

The race was close and [Lyndon B Johnson] was getting worried. Finally he told his campaign manager to start a massive rumor campaign about his opponent’s life-long habit of enjoying carnal knowledge of his own barnyard sows.

“Christ, we can’t get a way calling him a pig-fucker,” the campaign manager protested. “Nobody’s going to believe a thing like that.”

“I know,” Johnson replied. “But let’s make the sonofabitch deny it.”

Hunter S. Thompson, "Fear & Loathing: On The Campaign Trail ’72" (1973)

the LBJ campaign never formally denied the story.