Clinton has strong support from many of the "superdelegates".
Per CNN: "Superdelegates in the Democratic Party are typically members of the Democratic National Committee, elected officials like senators or governors, or party leaders. They do not have to indicate a candidate preference and do not have to compete for their position. If a superdelegate dies or is unable to participate at the convention, alternates do not replace that delegate, which would reduce the total delegates number and the "magic number" needed to clinch the nomination."
So while Obama has a slight edge in elected delegates, Clinton still has a sizable margin in overall delegates.
But with Rehnquist, even if Bush nominates someone who is solidly conservative, well so is Rehnquist, so that's a one for one swap. If you replace the more moderate O'Connor with someone like a Thomas or Scalia (which is the type of judges Bush wants)...