WASHINGTON (CNN) -- House Speaker Nancy Pelosi on Thursday requested that a federal grand jury be appointed to investigate whether a top White House official and former official should be prosecuted for contempt of Congress.

House Speaker Nancy Pelosi wants a grand jury to investigate two aides who've served President Bush.

In a letter sent to U.S. Attorney General Michael Mukasey and the U.S. attorney in the District of Columbia, Pelosi says White House Chief of Staff Josh Bolten and former White House counsel Harriet Miers were unresponsive to Congress' probe of the 2006 firings of U.S. attorneys.

Mukasey already has said he will not pursue charges against the two.

The White House has argued that contempt laws don't apply to the president or any of his staffers who invoke executive privilege.

In a statement Thursday, Justice Department spokesman Brian Roehrkasse said Mukasey had received the letter. He repeated the attorney general's belief that "long-standing Department precedent" prevents him from forwarding charges to a grand jury against White House staff if the president has told them to claim executive privilege.

"The attorney general is reviewing the referral and we anticipate that he will provide further guidance when that review is completed," Roehrkasse said.

"There is no authority by which persons may wholly ignore a subpoena and fail to appear as directed because a president unilaterally instructs them to do so," Pelosi wrote in her letters.

"Even if a subpoenaed witness intends to assert a privilege in response to questions, the witness is not at liberty to disregard the subpoena and fail to appear at the required time and place."

"I strongly urge you to reconsider your position and to ensure that our nation is operating under the rule of law and not at presidential whim," Pelosi wrote.

Earlier this month, the House voted to find Bolten and Miers in contempt of Congress and pursue charges against them.

The chamber's Republican minority staged a walkout before the vote.

A spokesman for House Republican leader John Boehner issued a statement calling Pelosi's request a "partisan political stunt" and "complete waste of time."

"This sort of pandering to the left-wing fever swamps of loony liberal activists does nothing to make America safer," spokesman Michael Steel said in the statement.

Miers refused to testify in the probe, which stemmed from the Justice Department's dismissals of federal prosecutors in eight cities. Bolten failed to produce documents in his possession, Pelosi's letter says.

The White House has insisted the firings were legal. But Democrats said the central questions behind the dismissals -- who decided the prosecutors should be ousted, and why -- remain unanswered.

Critics of the move have said prosecutors, including Republican appointees, who were not sufficiently supportive of Bush were targeted.

The House resolution authorizes the House Judiciary Committee to go to court to enforce the subpoenas issued to Miers and Bolten once the Justice Department, as expected, refuses.

The White House argues that forcing the aides to testify would violate the Constitution's separation of powers. E-mail to a friend

All About Nancy Pelosi • Joshua Bolten • Harriet Miers