At a crucial moment for his leadership, President Bush has picked a fight he was trying to avoid.

Last month, Bush was deemed too weak politically to risk a pitched battle with Democrats as he fills the seat of retiring Justice Sandra Day O'Connor. Now, after a failed Supreme Court nomination and the indictment of a senior White House aide, he has entered that fight after all.

Republicans and Democrats say that Bush had little choice.

"He decided to throw down the gauntlet on this one, which he clearly had to do to make his own party feel that there is something at stake here," said Anita Dunn, a Democratic campaign strategist.

Bush's selection of Judge Samuel A. Alito Jr. appears to have redeemed, at least in the eyes of conservatives, his campaign promise to nominate someone in the mold of Justice Antonin Scalia, the Supreme Court's most conservative member.

Their enthusiastic reaction yesterday was a powerful indication that Bush had achieved his immediate goals of shoring up his political base and relations with conservative activists and commentators whose unexpectedly harsh criticism helped turn his nomination of Harriet E. Miers into a debacle.

"Guess what? He learned from his mistake," said Richard Norton Smith, director of the Abraham Lincoln Presidential Library and Museum. "Presidents make mistakes, and the important ones learn from them."

Smith said it became clear during the Miers nomination that "the base was spoiling for a fight. My guess is, people in the administration will think that's not such a bad thing to have at the moment."

The nomination of Alito underscored the ability of any president, even one who is besieged, to shift attention from his problems. Days after Miers withdrew her name from consideration and the indictment of top White House aide I. Lewis "Scooter" Libby Jr., the focus has turned to a 55-year-old judge from New Jersey, his views on abortion and whether a bipartisan deal to block judicial filibusters will endure the coming clash.

"Right now, the [Alito] pick seems to be something that will be very popular with the base. The question is rounding up votes in the Senate," said former Republican Rep. Robert S. Walker, a Washington lobbyist.

Early indications are that the Alito nomination will draw more opposition than that of Chief Justice John G. Roberts Jr., whose confirmation was opposed by 22 Democratic senators and no Republicans, but there were no immediate signs that it would fail.

There were also no indications that the bipartisan deal against judicial filibusters might collapse. If it holds, Alito will need 50 votes to win confirmation in the Senate, where Republicans hold 55 seats.

"A lot of things aren't going well for this administration right now. If this goes well, it'll be perhaps the beginning of a fresh start," said Karlyn Bowman, a polling specialist at the conservative American Enterprise Institute.

A recent analysis by Bowman, which showed Bush losing support from conservatives, highlighted the stakes for Republicans, with elections next week for governor of New Jersey and Virginia, and the 2006 congressional elections a year away. Her study found that among conservatives, approval for the way Bush is doing his job slipped to 69 percent during the third quarter of this year from 76 percent in the first quarter.

Four weeks ago, when he nominated Miers, Bush went out of his way to speak about bipartisanship and thank Democratic senators for their input. Yesterday, he made no mention of bipartisanship or Democrats in introducing Alito to the nation.

But the rightness of his decision to nominate a certified conservative to the Supreme Court, at least from the White House perspective, could be glimpsed in the reactions of Republican senators, whose votes Bush had essentially lost as the Miers nomination was imploding.

Sen. Jeff Sessions of Alabama, who was noncommittal when Miers was picked, said Alito "has every quality necessary to be a great Supreme Court justice."

Virginia Sen. George Allen, a potential 2008 GOP presidential contender who had said Miers wasn't the best qualified person for the court, praised the "solid nomination" of Alito.

Sen. Trent Lott of Mississippi, who expressed misgivings about Miers, called Alito "a brilliant choice."