Why did President Bush pull such a quick switch yesterday and name Judge John Roberts to be chief justice of the United States? There are several theories, but here's the most sensible: Bush had always planned to make Roberts chief someday.

Recall that before Sandra Day O'Connor announced her retirement in July, much of official Washington was expecting that Chief Justice William Rehnquist would be the first to leave. The administration's extensive vetting was organized in large part to find a replacement for Rehnquist.

Roberts's combination of a conciliatory personality with firmly conservative convictions made him the perfect choice to lead the judicial revolution Bush would like to unleash. Successful chief justices need to be good politicians, and Roberts's political gifts were much in evidence this summer as he seemed to be charming his way toward relatively easy Senate approval.

But Rehnquist's passing and Bush's decision to withdraw Roberts's nomination as associate justice so he could name him chief create new political problems for both Bush and his appointee.

Until yesterday, even Roberts's staunchest critics judged their chances of stopping him for O'Connor's seat as bleak. Discussion among Senate Democrats revolved less around a strategy for blocking Roberts than on tactics that would make the best of his likely confirmation.

The debate focused on how many votes Democrats should try to muster against him -- to send a message for the big fight that would come over Rehnquist's job -- and whether senators from states where Bush had been popular should even consider voting no. Should Democrats try to look "moderate" by going along, or cast themselves as fighters for principle?

By proposing that Roberts lead the court, Bush has given the liberal groups that oppose the nomination (and Democratic senators inclined to join them) a chance to regroup and argue that this battle is no longer a practice session for the next round. This is the next round.

"Now that he's been nominated for chief justice, he's not a test case anymore," said a Senate Democratic staffer close to his party's discussions. "There's a difference between being one of nine and Number One of nine. And if he's confirmed, he's likely to hold the job for the next generation."

Fully aware that postponing confirmation hearings scheduled for today would give Roberts's opponents more time to organize, some Senate Republicans urged against any delay. Yesterday, it became clear that the hearings would be put off until either Thursday or next Monday. But the administration's allies are clearly wary of any further postponement.

Democrats, in turn, wanted more time to make the case that their requests for Roberts's past writings as an executive branch official -- documents the administration is holding back -- are more reasonable and appropriate than ever, "given the even greater importance of this new position," as Sen. Charles E. Schumer (D-N.Y.) put it yesterday.

Moreover, now that Bush will be naming not one but two justices, it becomes clear that the argument before the Senate is not primarily about one person and his qualifications, but about the future direction of the judiciary. Sen. Edward M. Kennedy (D-Mass.) was right to say that before the Senate acted on Roberts, "we should know whom the president intends to propose to nominate as a replacement" for O'Connor, since Americans were concerned with "the overall balance of their highest court."

Until yesterday's announcement, Roberts seemed likely to win on the basis of his personal attributes. By raising the stakes, Bush has given the Democratic opposition an opportunity to show some spine. He has reminded senators that this is a vote not just about a smart, affable lawyer, but also about principles that could steer the Supreme Court for 30 years.

There is no telling whether Bush's diminished standing from the disastrous failure of hurricane relief efforts may embolden Democrats to challenge the White House across a much broader front, including the future of the court. But in the statement he issued within hours of Roberts's new nomination, Kennedy wanted no one to miss that possibility. He pointedly referred to Hurricane Katrina as "a defining moment in our nation's history" and urged the president "to take this time to unite and heal the country."

Bush no doubt turned to Roberts as a safe harbor in the midst of the greatest political storm of his presidency, and Roberts may yet triumph. But the fierce winds that have buffeted Bush could imperil what once might have been an easy passage for a calm and collected nominee who, in his warm and witty way, would move the court and the country rightward.