A federal judge ruled Thursday that Bush administration advisers are not immune to congressional subpoenas.

The decision gives Democrats on Capitol Hill a major victory in their attempts to hamstring the Bush administration through a number of ongoing investigations.

In one of the probes, the House Judiciary Committee wants to question the president's chief of staff, Josh Bolten, and former legal counsel Harriet Miers, about the firing of nine U.S. attorneys. Congress has subpoenaed them both, and issued contempt citations.

But President Bush says the two are immune from such subpoenas and Congress can't force them to testify or turn over documents.

U.S. District Judge John Bates disagreed. He said there's no legal basis for that argument. He said that Miers must appear before Congress and, if she wants to refuse to testify, she must do so in person.

Click here to read U.S. District Judge John Bates' decision in the case.

"Harriet Miers is not immune from compelled congressional process; she is legally required to testify pursuant to a duly issued congressional subpoena," Bates wrote.

He said that both Bolten and Miers must give Congress all non-privileged documents related to the firings.

The Bush administration can appeal the ruling.

"This is very good news for anyone who believes in the Constitution and the separation of powers. This has consequences for other contempt citations pending before the House," House Speaker Nancy Pelosi, D-Calif., told reporters Thursday morning after learning of the decision.

And House Judiciary Committtee Chairman John Conyers, D-Mich., called the rulling "a ringing reaffirmation of the fundamental principle of checks and balances and the basic American idea that no person is above the law," adding that he expected Miers to testify before his panel in September.

Earlier this year, the full House voted to hold Bolten and Miers in contempt of Congress for failing to testify.

And just yesterday, the House Judiciary Committee voted to hold former White House adviser Karl Rove in contempt of Congress for failing to testify about the firing of the U.S. attorneys, which Democrats allege was done on the basis of political purposes.

House Minority Leader John Boehner, R-Ohio, downplayed the weight of the decision, saying the case "will go on until next year and be appealed and go through the courts and become a moot issue" after the Bush administration leaves office.

He also dismissed the case as "a political ruse by some Democratic Committee Chairman."

White House spokesman Tony Fratto said: "We're reviewing the decision. We will let you know if we have more comment."

The Associated Press contributed to this report.