WASHINGTON — Eight federal prosecutors were fired last year because they did not sufficiently support President Bush’s priorities, Attorney General Alberto Gonzales’s former chief of staff says in remarks prepared for delivery today to Congress.

“The distinction between ‘political’ and ‘performance-related’ reasons for removing a United States attorney is, in my view, largely artificial,” said Kyle Sampson.

The aide, who quit because of the furor over the firings, is to testify before the Senate Judiciary Committee. A copy of his prepared remarks was obtained yesterday by the Associated Press.

“A U.S. attorney who is unsuccessful from a political perspective … is unsuccessful,” Mr. Sampson said.

Democrats have described the firings as an “intimidation by purge” and a warning to remaining U.S. attorneys to fall in line with Mr. Bush’s priorities. Political pressure, Democrats say, can skew the judgment of prosecutors when deciding whom to investigate and which indictments to pursue.

Mr. Sampson maintained that adherence to the president’s and attorney general’s priorities was a legitimate standard. He strongly denied Democrats’ allegations that some of the prosecutors were dismissed for pursuing Republicans too much and Democrats not enough in corruption cases.

“To my knowledge, nothing of the sort occurred here,” he said. “As presidential appointees, U.S. attorneys serve at the ‘pleasure of the president’ and may be asked to resign for almost any reason, with no public or private explanation.”

Mr. Sampson’s testimony today is voluntary, though the committee chairman, Senator Leahy, told reporters he has kept a signed subpoena under lock and key in case Mr. Gonzales’s chief of staff backed out.

There was no indication of that happening. In his remarks, Mr. Sampson said he was pleased to appear and pledged to stay as long as necessary.

Nothing will stop the investigation, Mr. Leahy said yesterday — not even Mr. Gonzales’s resignation.

“In case anybody’s thinking of shortchanging it that way, I have a message for them: We’ll finish this investigation before we’ll have any confirmation hearings for a new attorney general,” said Mr. Leahy, a Democrat of Vermont. “I want to know what the facts were.”