KWAME HOLMAN: John Kerry has given his military record a principal role in his campaign for president. And at the Democratic Convention last month, Kerry featured prominently several veterans who attested to his bravery and leadership while commanding a swift boat in the Mekong Delta in Vietnam.But earlier this month, a group called Swift Boat Veterans for Truth launched a TV ad charging Kerry with lying about his tour in Vietnam that led to three Purple Heart combat medals. The group is funded in part by a wealthy Republican donor; the ad ran this month in three battleground states: Ohio,...