Washington ? Congress issued subpoenas Wednesday 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. Complying could 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 bread crumbs in this investigation have always led to 1600 Pennsylvania,” said House Judiciary Committee Chairman John Conyers, D-Mich. “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 Senate Chairman Patrick Leahy, D-Vt.

Leahy’s subpoena compels Taylor’s testimony on July 11, while Conyers subpoenaed Miers to appear the next day.

Both panels also subpoenaed White House documents relevant to the investigation.

The investigation by majority Democrats is fueled by their contention that Attorney General Alberto 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 Gonzales’ conduct, congressional Republicans have pointed out that U.S. attorneys are political appointees who can be hired and fired for almost any reason.