Congressional Democrats vowed to press ahead yesterday in their investigations into the White House’s role in the firings of nine U.S. attorneys last year and alleged politicization of the Justice Department’s hiring process, saying senior Bush adviser Karl Rove’s departure only increases their demand for information.

“The list of senior White House and Justice Department officials who have resigned during the course of these congressional investigations continues to grow, and today, Mr. Rove added his name to that list,” the Senate Judiciary chairman, Patrick Leahy, said yesterday. “There is a cloud over this White House, and a gathering storm. A similar cloud envelopes Mr. Rove, even as he leaves the White House.”

Mr. Rove yesterday said he would be leaving the White House at the end of the month. The surprise announcement came 11 days after he refused to testify before Mr. Leahy’s panel about his role in the dismissals of the federal prosecutors. Citing legal rulings from the Justice Department, White House counsel Fred Fielding early this month said in a letter to Mr. Leahy, a Democrat of Vermont, that President Bush was asserting “absolute immunity” for his advisers in barring Mr. Rove from testifying. Mr. Bush asserted that executive privilege because he wants to protect “the ability of future Presidents to ensure that the Executive’s decisions reflect and benefit from the candid exchange of informed and diverse viewpoints,” Mr. Fielding said in the letter. Mr. Fielding has offered to allow Mr. Rove and other current and former West Wing staff to be interviewed by staffs from the House and Senate Judiciary committees — behind closed doors and not under oath.

Legal experts were divided on whether Mr. Rove’s resignation heightens the likelihood that he will eventually testify under oath on Capitol Hill. Louis Fisher, a specialist on the separation of powers at the Congressional Research Service, said the legal claim of privilege will still extend to Mr. Rove after he leaves the White House because it is an assertion related to the duties he performed while in office. “I don’t think that the principle changes. What does change is, the White House doesn’t have that person by the neck,” he said.

Unless Mr. Rove’s loyalty were to change, which no one expects, Mr. Fisher said the Democrats need to uncover sharper examples of criminal wrongdoing in their investigations to bolster their claim that the need for this testimony outweighs the White House’s privilege claim.

But Charles Tiefer, a constitutional scholar at the University of Baltimore, said the reality is that former White House staff members have oftentimes testified on Capitol Hill even though they claimed immunity while they were serving in the West Wing. A former secretary of state, Henry Kissinger, after initially refusing to testify, appeared before several congressional committees examining alleged abuses of intelligence surveillance in the late 1970s. And Oliver North, while still on the national security staff in the White House in 1986, agreed to only appear in off-the-record meetings with the House Intelligence Committee, but once he left the staff later that year North met with the panel in full sessions discussing his role in the Iran-contra affair.

Mr. Tiefer said the Democrats need “some softening up” of Mr. Rove’s position, which could come over time. In many cases, the most often cited rationale for refusing to testify by White House staff is that it interferes with an official’s ability to advise the president, he said. Once the aide leaves the White House, that pressure can decrease. “He can no longer say that an appearance would compromise his ongoing White House duties. It becomes more and more untenable for him to refuse to show up,” Mr. Tiefer said.

Congress is in recess until after Labor Day, and Mr. Leahy is weighing his next step in the constitutional showdown with the White House over the testimony of Mr. Rove and other Bush aides. If Mr. Leahy asserts that the White House claim of immunity is not valid, the committee could vote on whether to hold Mr. Rove in contempt of Congress. The House Judiciary Committee has already approved a contempt citation for a former White House counsel, Harriet Miers, who refused to testify last month, and Chief of Staff Joshua Bolten, for refusing to turn over White House documents that were subpoenaed by the committee. A full House debate and vote is not expected until sometime in mid-September.

If the Senate were to vote out a contempt citation against Mr. Rove, it would be forwarded to the U.S. attorney for the District of Columbia, who, by law, would be required to impanel a criminal grand jury. But the Justice Department has said that Mr. Bush’s privilege claim is valid and therefore no federal prosecutor would pursue a contempt charge based on that privilege.