WASHINGTON — President Bush ordered his former counsel, Harriet Miers, to defy a congressional summons, even as a second former aide told a Senate panel yesterday that she knew of no involvement by Mr. Bush in the dismissals of eight federal prosecutors.

Contempt citations against both women were a possibility.

House Democrats threatened to cite Ms. Miers if she refused to appear as subpoenaed for a Judiciary Committee hearing today. The White House said she was immune from the subpoena and Mr. Bush had directed her not to appear, according to Ms. Miers’s lawyer. Democrats said her immunity ended when she left her White House job.

Across the Capitol, meanwhile, former White House political director Sara Taylor found out what Ms. Miers may already have known: It’s almost impossible to answer some committee questions but not others without breaching either the subpoena or Mr. Bush’s claim of executive privilege.

After first refusing to answer questions about Mr. Bush’s possible role in the firings, Ms. Taylor later told the Senate Judiciary Committee that she knew of no involvement by the president. Further, she said, she knew of no wrongdoing by administration officials in the controversy that has hobbled the Justice Department and imperiled Attorney General Gonzales.

The developments whipped across Washington as part of a broader dispute over the boundaries of Mr. Bush’s executive power and Congress’s oversight duty. Democrats, in control of Congress for the first time in a dozen years, are probing whether the White House ordered the prosecutor firings in ways that might help Republicans in elections.

The Bush administration acknowledges that the firings were clumsily carried out but insists no wrongdoing occurred. Mr. Bush has offered to allow his aides, including counselor Karl Rove, Ms. Miers and Ms. Taylor, to be interviewed by congressional investigators — but only in private and without a transcript.

Democrats on the committees rejected the offer and subpoenaed Ms. Miers and Ms. Taylor to appear this week, a possible foreshadowing of what’s to come for Mr. Rove.

In letters dated Tuesday, White House Counsel Fred Fielding told Ms. Miers’s lawyer that Mr. Bush had ordered her to stay away from today’s hearing.

“Ms. Miers has absolute immunity from compelled congressional testimony as to matters occurring while she was a senior adviser to the president,” Mr. Fielding wrote to Ms. Miers’s lawyer, George Manning. “The president has directed her not to appear at the House Judiciary Committee hearing on Thursday, July 12, 2007.”

Mr. Manning, in turn, notified the committee chairman, Rep. John Conyers, a Democrat of Michigan, and Rep. Linda Sanchez, a Democrat of California, chairwoman of the subcommittee on commercial and administrative law.

Mr. Conyers had previously said he would consider pursuing criminal contempt citations against anyone who defied his committee’s subpoenas.

“A refusal to appear before the subcommittee tomorrow could subject Ms. Miers to contempt proceedings,” Mr. Conyers and Ms. Sanchez, wrote back to Mr. Manning. “The subcommittee will convene as scheduled and expects Ms. Miers to appear as required by her subpoena.”

At the same time, the Senate Judiciary Committee chairman, Senator Leahy, held open the possibility of contempt proceedings against Ms. Taylor if she does not answer follow-up questions posed during his hearing yesterday.

“That’s a decision yet to be made,” Mr. Leahy said.

Ms. Taylor, eager to exhibit a willingness to answer questions but refusing to do so on many of them, exposed some details behind the firings.

“I did not speak to the president about removing U.S. attorneys,” she said under stern questioning by Mr. Leahy, a Democrat of Vermont. “I did not attend any meetings with the president where that matter was discussed.”

When asked more broadly whether Mr. Bush was involved in any way in the firings, Ms. Taylor said, “I don’t have any knowledge that he was.”

She said she did not recall ordering the addition or deletion of names to the list of prosecutors to be fired. Ms. Taylor said she had no knowledge that Mr. Bush was involved in the planning of whom to fire, an assertion that echoed previous statements by Mr. Gonzales, his former chief of staff, Kyle Sampson, and Deputy Attorney General Paul McNulty. Ms. Taylor disputed Sampson’s account that she wanted to avoid submitting a new prosecutor, Tim Griffin, through Senate confirmation.

“I expected him to go through Senate confirmation,” Ms. Taylor said under questioning by Senator Feinstein, a Democrat of California.