Bush could either nominate Roberts to be Chief Justice or elevate Scalia to Chief Justice concurrently with Roberts’ nomination. The Court would not be a justice short for the two to three month period that vetting, nominating and confirming another Supreme Court nominee would take.
The suggestion would be difficult for Bush to refuse and would discredit the inevitable claim that Democrats must rush through the second nomination, because the court is still one justice short of a full bench.
[Update: 9/05] Supreme Court Picture Gets Complex:
It would be easy for the president to switch gears on the Roberts nomination. He could withdraw his nomination to be an associate justice and resubmit his nomination to be chief justice, lawyers and judiciary committee staffers said Sunday.
“The president can do that without almost any uncertainty because he knows how the Roberts nomination [for associate justice] has been received,” said Brad Berenson, a Washington lawyer who worked for the Bush White House. “Barring something unforeseen, he would be easily confirmed for chief justice.”
Berenson added: “This is the only scenario in which the court could begin the term with a full complement of justices, since Justice O’Connor pledged to stay on until her successor is confirmed.”