Take, for example, the court's recent rulings on executive power in the war on terror. It has repeatedly rejected Bush's policies for detaining terror suspects at Guantanamo Bay, Cuba, ruling most recently in June that suspects have a right to challenge their detention in federal court. That landmark decision was 5-4, with Kennedy joining the four liberals to cast the key vote.
Or abortion. Right now, the court is at least one vote short of overturning Roe v. Wade. Only Scalia and Thomas have said they would overturn it; Roberts and Alito have not yet ruled on the issue. But Kennedy, who has supported abortion restrictions, has nonetheless refused to overturn Roe and helped send the issue back to the states to decide. One new justice could make a difference.
Obama, on the other hand, voted against Roberts and Alito. He has said he would nominate justices who believed the court should take a more active role in protecting people's rights, and he often uses the case of Lilly Ledbetter, an Alabama woman who lost a pay discrimination claim on a five-four vote, to make his point.
But Obama would probably just be holding a liberal place card. If he replaces Stevens, Ginsburg or Souter, it won't change the balance of the court.
Here, though, history can be a cautionary guide. In 2005, court watchers expected Chief Justice William Rehnquist to give Bush his first Supreme Court vacancy. While Bush would have an historic opportunity to nominate the next chief justice, he wouldn't be changing the court's direction -- he would just be replacing a conservative with a conservative.
But moderate conservative Justice Sandra Day O'Connor delivered a bombshell: She announced her retirement first, giving Bush the opportunity to change the court, which he did when he replaced O'Connor with the more solidly conservative Alito.
If a conservative justice delivered a similar bombshell in the Obama administration, President Obama will then have an historic opportunity of his own to turn the court's direction. With Democrats controlling the Senate, Obama could quickly undo Bush's legacy.
By replacing a conservative justice, Obama would turn the court to the left with his nomination. That would make the court more liberal than it's been in decades -- and more receptive to affirmative action, discrimination claims, drawing firm lines between church and state and numerous other social issues where conservatives have held sway.