WASHINGTON, Aug. 6 - A commercial by a group of veterans accusing Senator John Kerry of exaggerating his decorated Vietnam War record was strongly challenged Friday by Democrats as new questions arose about the claims of one of the men calling Mr. Kerry a liar in the advertisement. The Boston Globe quoted the man, Lt. Cmdr. George Elliott, retired, who as Mr. Kerry's commander in Vietnam had recommended him for a Silver Star, as recanting a recent sworn statement that he did not believe Mr. Kerry deserved the decoration because of allegations that he earned it by shooting a fleeing,...