Republican U.S. presidential candidate John McCain opened a new line of attack against Democratic front-runner Barack Obama on Thursday, taking aim at Obama's lack of military service.McCain, a former Navy pilot and prisoner of war in Vietnam, struck back after Obama criticized McCain on the U.S. Senate floor in Washington for opposing legislation that would give expanded benefits to military veterans. "It is typical, but no less offensive, that Senator Obama uses the Senate floor to take cheap shots at an opponent and easy advantage of an issue he has less than zero understanding of," said McCain, who was not present for the Senate vote because he was campaigning in California.