WASHINGTON — Congress issued subpoenas yesterday for former presidential counsel Harriet Miers and political director Sara Taylor, reaching directly inside the White House for the first time in the probe of the firings of federal prosecutors.

The Bush administration appeared in no hurry to encourage the pair to testify, as the subpoenas demanded, not wanting to set a precedent for testimony by another adviser not yet on the subpoena list: presidential counselor Karl Rove.

The Democratic chairmen of House and Senate committees implicitly threatened a constitutional showdown if the White House does not comply with the subpoenas — or strike a deal.

“The breadcrumbs in this investigation have always led to 1600 Pennsylvania,” said the House Judiciary Committee chairman, Rep. John Conyers, a Democrat of Michigan. “This investigation will not end until the White House complies with the demands of this subpoena in a timely and reasonable manner so that we may get to the bottom of this.”

“The White House cannot have it both ways — it cannot stonewall congressional investigations by refusing to provide documents and witnesses while claiming nothing improper occurred,” added the Senate chairman, Senator Leahy, a Democrat of Vermont.

White House officials pointed out that White House Counsel Fred Fielding already has offered a compromise by suggesting that Ms. Miers, Ms. Taylor, Mr. Rove, and their deputies be interviewed by committee aides in closed-door sessions, without transcripts. Messrs. Leahy and Conyers have rejected that offer.

“The committees can easily obtain the facts they want without a confrontation by simply accepting our offer for documents and interviews,” White House spokeswoman Dana Perino said yesterday. “But it’s clear that Senator Leahy and Representative Conyers are more interested in drama than facts.”

Mr. Leahy’s subpoena compels Ms. Taylor’s testimony on July 11, while Mr. Conyers subpoenaed Ms. Miers to appear the next day. Both panels also subpoenaed White House documents relevant to the investigation.

In a statement, Ms. Taylor’s lawyer did not rule out her appearance but suggested it depends on agreement between the White House and Congress.

“Ms. Taylor takes her responsibilities as a citizen very seriously and she is hopeful the White House and the Congress are quickly able to work out an appropriate agreement on her cooperation with the Senate’s proceedings,” said her attorney, W. Neil Eggleston.

Ms. Miers did not immediately respond to a request for comment.

The investigation by majority Democrats is fueled by their contention that Attorney General Gonzales has, in effect, allowed the White House to make major political decisions for the traditionally independent Justice Department. The firings of eight federal prosecutors over the winter, Democrats say, was an example of improper political influence.

Though widely unhappy with Mr. Gonzales’s conduct, congressional Republicans have pointed out that U.S. attorneys are political appointees who can be hired and fired for almost any reason.

Some in the GOP have said it would be improper if any of the eight were forced out because they did not pursue corruption investigations that might have helped Republicans. Mr. Gonzales and the White House have denied that took place, but the attorney general also told a Senate committee dozens of times that he could not recall key details of the firings.

E-mails released by the Justice Department show that Mr. Rove, Ms. Miers, Ms. Taylor, and other White House officials were connected to the decision-making process that led to the firings.

Ms. Miers left the White House January 4. Ms. Taylor’s last day was May 30.

If the showdown between the White House and Congress is not resolved, the matter could end up with House and Senate contempt citations and a session in federal court.

Senator Specter, a Republican of Pennsylvania, pointed out that a resolution without a constitutional showdown is in both sides’ interests.