If President Bush has to replace cancer-stricken Chief Justice William Rehnquist, sources close to the court say the White House is leaning toward moving up Associate Justice Clarence Thomas rather than naming the more prickly Antonin Scalia. To replace Thomas, President Bush could well nominate the brilliant Miguel Estrada, who was blocked from the Court of Appeals by obstructionist Tom Daschle. Estrada is now in private law practice.