Ok, it’s not original, but it’s still a very good reason.

Chief justice William Rehnquist–Age 78
Justice Sandra Day O’Connor–Age 73
Justice John Paul Stevens–Age 83

Conservative presidents do not have a very good record in the past 50 years of appointing true conservatives to the Supreme Court. However, liberal Democrats have a very good record of appointing liberal justices to that court. So, worst case, it’s a choice between a chance of getting a truly conservative Supreme Court justice or the near-certainty of getting yet another liberal appointed to the Court. And I actually believe that the words of the Constitution, although not on par with Holy Scripture, mean something. I think that the only way to change the Constitution is to amend it, not to read it as a “living document” that means anything some judge thinks it ought to mean. So I’ll be voting for GWB who might have a small chance of appointing judges to the bench who believe in interpreting the Constitution to mean what it actually says.