His voting record. I couldn't imagine Bush attacking Kerry's military record, that would have disatrous consequences.
His voting record is terrible. If more Americans actually looked at it we would be looking at a 6 point Bush victory!
Bush has excuses for his record in office, even if they aren't accurate. Kerry has no excuses for his votes.
Many of the defense cuts Kerry voted for were supported by many other Republicans, including Bush's father. I think Kerry will actually win the debates, but Bush will have better talking points, and will be the percieved winner.