WASHINGTON (Reuters) - Democratic presidential candidate John Kerry (news - web sites) returned to Congress on Thursday for a series of party unity meetings and slammed a "Republican attack squad" that he said specializes in destroying foes. Kerry, facing a flood of Republican criticism for characterizing his critics in the other party on Wednesday as a "crooked, lying group," refused to apologize and amplified his charges. "There is a Republican attack squad that specializes in trying to destroy people and be negative," Kerry told reporters after having lunch in the Capitol with his Senate Democratic colleagues. "I think the president...