The death of Justice Antonin Scalia resulted in a vacancy on the Supreme Court. Republican Senate leaders have unequivocally stated that they will oppose any nominee President Obama puts forward to fill this spot.
The Constitution states that "The President shall have Power to fill up all Vacancies that may happen during the Recess of the Senate, by granting Commissions which shall expire at the End of their next Session." Art. II, §2, Cl. 3. The Senate is currently in recess and the important work of the Supreme Court cannot grind to a halt because of partisan intransigence.
Justice John Paul Stevens was nominated by a Republican president, confirmed 98-0, and was the Court's third-longest serving justice. He has earned his colleagues' respect and would be prepared to start on day one.