WASHINGTON – President Bush moved boldly to shift the Supreme Court to the right Tuesday night by selecting U.S. Appeals Court Judge John Roberts Jr. to succeed Justice Sandra Day O’Connor. But in choosing a jurist with establishment credentials and bipartisan allies, Bush was also looking for a nominee who could win confirmation with some Democratic votes.

Bush passed over a number of highly conservative judges whose nominations would have been seen as far more polarizing than Roberts’. Given that this was the first but probably not the last Supreme Court vacancy he will be asked to fill, Bush sought a less confrontational approach with the Senate than he has adopted with his lower-court nominations.

Roberts faces a potentially contentious confirmation battle in any case, given the significance of O’Connor as the swing vote in many of the court’s most important cases.

But Senate Democrats reacted cautiously, saying only that there were many questions they wanted Roberts to answer during his confirmation hearings. Privately, they were being urged to keep their powder dry until a fuller vetting of Roberts’ record both as a judge and a lawyer is completed later this summer.

That may have been as much tactical as substantive, given the fact that Senate Democratic leaders had urged their colleagues not to overreact initially, no matter who Bush nominated. Later, they plan to press for access to records relating to Roberts’ service in the Reagan administration, and if denied will turn up the heat.

Whether Democratic leaders carry through with threats to filibuster a Supreme Court nominee they regard as too conservative is the more important matter. Given Roberts’ background and demeanor, that is now a much more difficult choice.

For the White House, Roberts appears to be the ultimate confirmable conservative. As a replacement for O’Connor, a centrist who voted to uphold abortion rights and affirmative action, he would probably move the court’s overall balance to the right. But he would do so without some of the verbal pyrotechnics that have characterized the opinions of conservative Justice Antonin Scalia.

As a lifelong Republican, Federalist Society member and veteran of the Reagan and first Bush administrations, the 50-year-old has presumably established his credentials for the right.

But his rhetoric is cool, earning him many friends and few outspoken enemies. His legal abilities are widely acknowledged to be excellent. And he has assembled a paper record that presents no undeniable proof of personal views that could be attacked as extreme.

One of Roberts’ key advantages is his strong reputation among fellow members of the bar, including many Democrats. Those relationships figure to earn him the support – or at least the neutrality – of a constituency that might otherwise be well-placed to make the confirmation process difficult for the administration.