Barack Obama ratcheted up criticism of Republican John McCain, comparing him with the unpopular Bush administration and continuing an effort to make the former war hero's opposition to a college aid bill for military veterans a campaign issue. McCain was a Navy fighter pilot who was shot down and spent nearly six years imprisoned in Vietnam. He has stressed his impressive military record and Obama's lack of one as Obama appears close to beating Democratic rival Hillary Rodham Clinton for the presidential nomination. On Saturday, Obama appealed to military veterans, saying 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. "Now, let me be clear: No one can dispute John McCain's love for this country or his concern for veterans. But here's what I don't understand. I don't understand why John McCain would side with George Bush and oppose our plan to make college more affordable for our veterans," Obama said. "George Bush and John McCain may think our plan is too generous. I could not disagree more."