WASHINGTON - Three campaign finance watchdog groups filed a complaint Tuesday accusing a group of Vietnam veterans of violating the campaign finance law by airing an ad that challenges Democratic presidential nominee John Kerry (news - web sites)'s military record. Democracy 21, the Center for Responsive Politics and the Campaign Legal Center argue that the ad by Swift Boat Veterans for Truth violates a federal ban on the use of unlimited donations, often referred to as "soft money," to influence federal elections. In the ad, the group accuses Kerry, a decorated Vietnam veteran, of lying about his war record and...