BOSTON - John Kerry fought back Thursday against campaign allegations that he exaggerated his combat record in Vietnam, accusing President Bush of using a Republican front group “to do his dirty work” and challenging Bush to debate their wartime service records. “Well, if he wants to have a debate about our service in Vietnam, here is my answer: Bring it on,” said the Democratic presidential candidate, reviving an old war and campaign slogan amid strong urging from party leaders for him to respond to two-week-old GOP assertions. As Kerry denounced the criticism as “lies about my record,” aides privately acknowledged...