1. Republican presidents nominate judges that are liberal and conservative. (see: Ronald Reagan's Anthony Kennedy and Antonin Scalia or George H.W. Bush's David Souter and Clarence Thomas)
2. Democrat presidents ALWAYS nominate judges that are liberal (see: Bill Clinton's Ruth Buzzi Ginsburg and Stephen "let's decide our laws by imitating foreign countries" Breyer)
3. NOTHING in the Constitution prevents a firmly controlled Democrat congress and a President Hussein Obama from deciding to expand the size of the Supreme Court from 9 to 11 or 13 justices, then loading it with liberal hacks...thus nullifying the firm conservatives put on the court by President Bush 43.
After all these considerations, I will be voting for McCain. He isn't a guarantee of a more conservative court, but he IS necessary for one.
People often forget one very significant check on the government's power: to be effective, it must be seen as legitimate. Today, the Supreme Court has an aura of legitimacy far beyond anything it deserves. Were Obama to pack the court with Bill and Hillary, Gore and Kerry, that could prompt a crisis of legitimacy(*); since I don't see any path to lawful government without first going through a crisis of legitimacy, having the Democrats trigger the crisis would probably not be a bad thing.
(*) A crisis of legitimacy is the turning point between civil war and lawful government; the more widely the government is seen to be illegitimate, the greater the likelihood of restoring lawful government without civil war.