Barack Obama ratcheted up criticism of Republican John McCain, linking him with the deeply unpopular Bush administration and reinforcing a bid to make the former war hero's opposition to a college aid bill for military veterans a major campaign issue. Obama appealed to military veterans, saying Saturday while campaigning in Puerto Rico that he cannot understand why McCain opposes legislation that would provide college scholarships to people who have served in the US military. Puerto Rico holds its primary on June 1, but does not vote in the general election. The Arizona senator opposes the scholarship measure, as does the Pentagon, because it applies to people who serve just three years. He fears that would encourage people to leave the military after only one enlistment at a time when the US is fighting two wars and trying to increase the size of the Army and Marine Corps.