Weakened by scandal, Iraq and the failed nomination of Harriet Miers, President Bush offered up a Supreme Court nominee yesterday guaranteed to rally his fractured Republican base. But the choice of Judge Samuel A. Alito Jr. appears certain to produce an angry ideological battle with the Democrats that will dominate the country's politics heading into next year's midterm elections.

The Alito nomination instantly brought a fresh eruption of the partisan warfare that has defined the Bush era, and it set the stage for the battle many predicted four months ago when Justice Sandra Day O'Connor, the court's swing vote on many key issues, announced her intention to retire.

With the ideological balance of the court now in question, partisans on the left and right moved swiftly to the barricades. Senate Democrats declined to rule out a filibuster to block Alito, and Republicans renewed talk of invoking the "nuclear option" if necessary to prevent one.

Whether the upcoming battle, which is likely to focus heavily on the divisive issue of abortion, ultimately helps a president whose approval ratings are scraping 40 percent, and whose support among moderates and independents has plummeted even lower, is an open question -- and one hotly debated among strategists yesterday.

Given the state of his presidency and party, Bush may have had no other choice than to name a Supreme Court candidate who would help to heal the divisions within the GOP coalition, even at the risk of further alienating voters in the center. Democrats were convinced the choice would move Bush's image irrevocably to the right, but some Republicans said this is exactly the kind of fight that could help turn around Bush's troubled presidency.

"The dispute over Miers's nomination shows that they're not adept at dealing with discontent on their own side," said Ron Klain, former chief of staff to Vice President Al Gore. "This is a president who has never governed from the center and who is clearly uncomfortable with the crosswinds that come with that. So they went back to familiar turf. I'm not sure they can afford to do that, but they lack the dexterity to do something different."

But Republicans said the only sensible recovery strategy for the embattled president begins with putting his own coalition back together. After months of bad news, the Republicans need a symbol around which to unify and for conservatives, Alito and changing the court may be precisely the answer.

Republican Vin Weber, a former representative from Minnesota, said a major fight with Democrats was inevitable after the Miers withdrawal and that Bush was wise to find a nominee who would be received enthusiastically by party conservatives. In the long run, he predicted, the fight would help Republicans more than Democrats.

"The Democrats don't need rallying right now and the Republicans do," he said.

Sen. Lindsey O. Graham (R-S.C.) argued that the confirmation battle over Alito will give Republicans an opportunity to put some of their current troubles into the background and to engage the Democrats on a debate over judicial philosophy and restraint that will win support not just among conservative activists but the moderate middle of the electorate.

"Liberal judicial activism is not in fashion in America. Having a fight over conservatism in terms of judging will put us closer to the middle than our friends on the Democratic left," Graham said. "We're being defined by our mistakes and our problems. This fight will give us a chance to talk about what they [Democrats] want if they're in charge and in that way it will be good for the Republican Party."

Charlie Black, a veteran GOP strategist and outside adviser to the White House, said Democrats may overestimate how much middle-of-the-road voters will be drawn to an ideologically polarizing debate over Alito's views. "I think average Americans -- moderates, independents -- look at the qualifications and credentials first, then will want to get to know the person better through the hearings," Black said.

Noting that new Chief Justice John G. Roberts Jr. proved not to be a polarizing figure because of his demeanor and intellect, Black said, "People who know Alito think the same thing might happen."

With years on the appeals court filling out a top-tier legal background, Alito's credentials will be hard for Democrats to challenge. And his demeanor is said by most who know him to be smoother and less confrontational than that of Justice Antonin Scalia, a hero of the right.

Even so, Alito's paper trail is far longer and more provocative on the most controversial social issues than was Roberts's. Liberal constituency groups immediately announced their opposition to Alito yesterday, and Senate Democrats, including some who voted for Roberts, issued tough, skeptical statements in response to his nomination.

Sen. Patrick J. Leahy (Vt.), the ranking Democrat on the Senate Judiciary Committee and a supporter of the Roberts nomination, called the president's choice "needlessly provocative," saying the president rewarded a narrow wing of the Republican Party rather than the country as a whole.

If GOP conservatives were energized by Bush's choice, Republican moderates in the Senate found themselves in an uncomfortable position. Sen. Lincoln D. Chaffee (R.I.) issued a statement outlining a string of concerns about Alito's record. Other moderates remained silent. Their votes will be the target of fierce competition between the White House and the Democrats, as will the votes of the seven Democrats in the bipartisan Gang of 14, whose compromise agreement in the spring averted a showdown over the future of Senate filibusters on judicial nominees.

"If you lock your moderates down, the likelihood of filibuster becomes less likely," one GOP lawmaker said yesterday. If Democrats "sense weakness on our side, then this guy is in trouble. The first goal is to lock down your moderates and then work on the seven Democrats in the Gang of 14 and create a comfort level with both."

Weber said a successful fight to confirm Alito would leave Bush and the Republican Party in "dramatically better shape" by early next year. But Democrats said they can foresee no significant risk in waging an all-out battle, which they believe will leave the Republicans further to the right than before, even if Alito wins confirmation. That assures the kind of confrontation the country has not seen over a high-court nomination since Bush's father picked Clarence Thomas in 1991.

Judge Samuel A. Alito Jr., left, appears at a news conference with Senate Majority Leader Bill Frist (R-Tenn.) after his nomination to the Supreme Court, which provoked skepticism among Democrats but applause from Republicans.