Setting up what could be the boldest challenge yet to the Bush administration’s drive to pack the nation’s courts with conservative judicial activists, Senate Democrats have signaled that they will mount a filibuster to block a Senate vote on the nomination of Bush favorite Miguel Estrada to serve on the powerful U.S. Circuit Court of Appeals for the District of Columbia.

The administration and Republican operatives in Washington and around the country have waged a fierce campaign to win Senate approval for Estrada, a former solicitor general who is a favorite of movement conservatives and is widely viewed as a likely contender for a future nomination to the U.S. Supreme Court. As recently as Tuesday, as the Senate entered the third day of deliberation on the nomination, the White House issued a statement from Bush demanding a quick “up or down vote on the Senate floor.” When he learned of the decision by Democrats to filibuster, Bush grumbled about how “a handful of Democrats in the Senate are playing politics with his nomination, and it’s shameful politics.”

But it is not just “a handful of Democrats.” Senate Democratic leaders say that more than 40 members will join efforts to block a vote. The delay, Democrats say, will extend at least until the White House releases information regarding Estrada’s legal views. That information was repeatedly requested by Democrats on the Senate Judiciary Committee prior to the panel’s 10-9 vote in late January to recommend approval of Estrada. The committee vote split along party lines, and was one of the first indicators that the new Republican leadership of the committee and the Senate would seek to force votes on even the most controversial judicial nominees.

Some Democratic senators have indicated that they would be inclined to vote against the Estrada nomination based on what is already known regarding the former solicitor general’s right-wing positions on civil rights and corporate power issues. But Senate Minority Leader Tom Dacshle, D-South Dakota, said the primary focus of any filibuster would be to force the administration to release memoranda that the nominee wrote while he worked in the office of the solicitor general in the Justice Department.

“Until that information is provided, we will not be in a position to allow a vote to come to the Senate floor,” Daschle told reporters after a mid-day meeting with Democratic senators at which a decision was made to try and block a vote on Estrada’s nomination. “It is critical that the administration recognize the importance of this information and the importance of the constitutional obligation (to review judicial nominations) that we hold very seriously.”

Daschle said that, after meeting with key senators, he is certain that Democrats have more than enough support to block a vote with a filibuster. Under Senate rules, 60 votes are needed to cut off a filibuster — where members who oppose a bill or nomination speak continually against it and employ other procedural moves in order to delay action — and force a final vote.

No one knows how the filibuster threat will play out, especially with the Senate preparing for a recess next week. Some Senate Republicans threatened to keep the chamber in session until a final vote is taken.

“If they want to stay through the weekend, we’ll stay through the weekend,” said Senate Majority Leader Bill Frist, R-Tennessee.

Republicans still hope to swing enough Democrats to their side to gain the 60 votes needed to break a filibuster and force a vote.A few Democrats, including influential Louisiana Senator John Breaux, have indicated that they will side with Republicans and support Estrada. And Judiciary Committee chair Orrin Hatch, R-Utah, has sought to pressure more Democrats to get in line behind a prominent Hispanic nominee. But those efforts were undermined when the Puerto Rican Legal Defense and Education Fund and the Southwest Voter Registration and Education Project joined the Congressional Hispanic Caucus in urging rejection of Estrada’s nomination.

Opposition to the nomination from leading civil rights groups — especially those with large Latino constituencies — is certainly influential with Democrats and some Republicans. Ultimately, however, the tipping point for Senate Democrats appears to have been Estrada’s stonewalling at hearings before the Judiciary Committee and the White House’s refusal to release the memoranda that effectively form the only official record of Estrada’s views.

“By remaining silent Mr. Estrada only buttressed the fear that he’s a far-right stealth nominee, a… candidate who will drive the nation’s second most important court out of the mainstream.”explained Senator Chuck Schumer, D-New York, a key Judiciary Committee member.

Nan Aron, president of the Alliance for Justice (www.afj.org and www.independentjudiciary.com), a national coalition of more than 60 organizations that advocates for an independent judiciary, says the decision of most Democratic senators to work to block Estrada is a major breakthrough. But, she adds, “The administration is going to put a lot of pressure on swing Democrats to break with their leadership and support Estrada. Our job in the next few weeks is to tell the Democrats to hold firm, and to start putting pressure on moderate Republicans. This is a fight that I think we can win, but there is no question that the fight is a long way from finished.”