President Bush finds himself in the unusual position of fending off conservative critics in the wake of his nomination of White House Counsel Harriet Miers to replace retiring Supreme Court Justice Sandra Day O'Connor.

Conservatives have long been the president's most loyal supporters. But they are split over his decision to nominate longtime aide Harriet Miers to the high court, with some activists questioning her judicial qualifications and whether she holds conservative views.

"This is a leap in the dark for conservatives when the president could have appointed individuals who had these demonstrated qualities both on the court and off the court," said Pat Buchanan, former Republican presidential contender turned conservative commentator on ABC television.

Many conservatives see a major opportunity to shift the balance of power on the closely divided Supreme Court to the right by replacing moderate Justice Sandra Day O'Connor with a proven conservative.

The concern about Ms. Miers has spread to conservative talk radio programs, including one hosted by Rush Limbaugh, a longtime defender of the president and Republicans in Congress. "The idea that we now have to roll the dice and wait a number of years to find out if this one works out when it is not necessary," he said.

"I am confident that she has a conservative judicial philosophy that you will be comfortable with, Rush," replied Vice President Dick Cheney on Limbaugh's program.

Harriet Miers must be confirmed by a majority vote of the Senate before she can replace Justice O'Connor.

But some Senate Republicans are expressing doubts that she is the best-qualified conservative available for the Supreme Court vacancy.

"So you look at this one and you say, well, why did we go here when you had so many other opportunities?," said Sen. Sam Brownback of Kansas, a member of the Senate Judiciary Committee, the panel that will conduct the confirmation hearings for Ms. Miers.

President Bush sought to reassure conservatives during a recent White House news conference. "There should be no doubt in anybody's mind what I believe the philosophy of a judge [should be]. Harriet Miers shares that philosophy. I know her well enough to be able to say she is not going to change," he said.

Some Senate Republicans are now speaking out on behalf of Harriet Miers, trying to reassure conservatives that she is qualified for the Supreme Court and that she was not picked merely for her longtime loyalty to the president.

"They will see the intellect, they will see the wisdom, the depth of experience, the judicial temperament that we all want in a Supreme Court nominee," said Texas Republican Sen. Kay Bailey Hutchison.

For the time being, opposition Democrats are saying little about the Miers nomination, preferring to let the debate among conservatives play out.

New York Democrat Charles Schumer, a member of the Senate Judiciary Committee, says the president is trying to please different constituencies with the Miers nomination. "On the one hand, not alienate the extreme wing of his party. And on the other hand, not alienate the rest of America. He did thread that needle with [Chief Justice] John Roberts. But whether he can do it again with Harriet Miers remains to be seen," he said.

Democrats promise intense questioning of Harriet Miers in the confirmation hearings, as they did during the hearings for Chief Justice John Roberts.

Conservatives are trying to avoid a repeat of what happened to the president's father, the first President Bush, when he nominated David Souter to the Supreme Court in 1990.

Justice Souter turned out to be far more liberal than conservatives expected and they have demanded proven conservative candidates for the court ever since.

"Those who think that, one, the president should have put a vociferous conservative, one who had been on the record for many years, out there, and worries that someone like [Harriet] Miers, who does not have much of a record, would move like a Justice [David] Souter to the left. You will hear some of that but I think at the end of the day the votes in the Senate, almost all of them on the conservative side, will be for her," said John Fortier, a political expert at the American Enterprise Institute in Washington.

Senate Republican leaders hope to hold a confirmation vote for Harriet Miers by late November. Until she is confirmed, Justice Sandra Day O'Connor remains on the Supreme Court.