For the younger folks: Kerry got some seriously badass medals for his military service in Vietnam. People who had been on the same boat as he did, but at a different time, worked with the Republican party to claim that he hadn’t really done what he did. The press ate this up, and distributed it widely, costing him public support and likely the election.
Adam Kinzinger (Republican) was in the National Gaurd and here he explains in clear terms why this swiftboating attempt is mendacious.
Edit: here is Lawerence Odonnell tears into his own network for a similar sin.