Here's the back of the flier ... essentially they are arguing that McCain betrayed his country. ... Or, more specifically, that he gave the Vietnamese information about U.S. flight paths.
First Read reports the group is headed by Jerry Kiley, who also ran a similar effort against John Kerry in 2004.
The Fix reports on McCain's efforts to fight back:
Even before the Michigan primary voting ends, surrogates for Sen. John McCain (Ariz.) organized a conference call with reporters to push back against an incendiary flier that was passed out to members of the South Carolina media today.
The flier, which came from a group called "Vietnam Veterans Against John McCain" and attacked the Arizona Senator's time spent as a prisoner of war in Vietnam, was roundly condemned by a group of McCain supporters that including former P.O.W. Orson Swindle as well as South Carolina House Speaker Bobby Harrell and state Attorney General Henry McMaster.
... It should be noted that McCain, Huckabee, and Romney are all reporting these sorts of nasty attacks in the state. Remembering what happening to him in 2000, McCain has organized what he calls a "Truth Squad" to fend-off these accusations. Right now, McCain and Huckabee are running neck-and-neck in the Palmetto state. This will likely continue to be a nasty race ...