The most famous example of this from the 2004 presidential campaign. Democrat John Kerry, a decorated Vietnam war veteran, was attacked with a barrage of TV ads by a shady group called "Swiftboat Captains for Truth", who questioned, without evidence, Kerry's three medals -- two bronze stars and one silver star. His opponent, George W. Bush, served in the Texas Air Force Reserve, largely believed to be a dodge to avoid service in the war.
https://en.wikipedia.org/wiki/Swift_Vets_and_POWs_for_Truth?wprov=sfla1