SPRINGFIELD, Ill. - Illinois lawmakers could be forced to build their impeachment case against Gov. Rod R. Blagojevich on a raft of relatively small grievances, rather than the blockbuster Senate-seat-for-sale allegations, for fear of undermining federal prosecutors' criminal investigation.

Members of the state House impeachment committee said yesterday that they would do nothing that would interfere with the investigation by U.S. Attorney Patrick Fitzgerald. If Fitzgerald asks lawmakers not to interview certain witnesses, they will abide by that, they said.

"We do not want to get in the way of the United States attorney doing the work he does, and so whatever he says about where we can and can't go in our investigation, we are going to just simply say, 'Fine, sir,' " said State Rep. Lou Lang, a suburban Chicago Democrat.

The committee would then probably emphasize some lower-profile allegations of misconduct against Blagojevich. Among them: defying the legislature, failing to honor reporters' Freedom of Information requests, and trading state jobs and contracts for campaign contributions.

The latest complications, along with other developments, suggest it could be more difficult to dislodge Blagojevich than it appeared just a week ago. On Wednesday, the Illinois Supreme Court rejected a request to declare him unfit to serve, and Blagojevich's attorney made it clear that the governor was not going down without a fight.

The panel adjourned until Monday, when members hoped to discuss any guidelines or restrictions Fitzgerald may place on them.

The federal prosecutors' case could be severely undermined - or at least greatly complicated - if Illinois lawmakers compel certain witnesses to testify under a grant of immunity. Following the Iran-contra scandal of the 1980s, the convictions of Oliver North and John Poindexter were thrown out after the courts determined that the cases against them were built too much on testimony they gave to Congress under a promise of immunity.

The impeachment committee sent Fitzgerald a letter yesterday formally asking for information about people mentioned by pseudonyms in the criminal complaint, and requesting his guidance on who could be called to testify. Fitzgerald refused to comment.

Committee members said the criminal charges against Blagojevich would still play a part in the impeachment proceedings. If nothing else, lawmakers will be able to use the 76-page federal criminal complaint, which includes sworn statements from the FBI and damning excerpts from the governor's wiretapped conversations.

Committee chairwoman Barbara Flynn Currie, a Chicago Democrat like the governor, noted that even before Blagojevich's arrest last week, some lawmakers were calling for his impeachment.

"We've got plenty of evidence out there of questionable activity on the part of the governor," she said.

The governor was arrested on charges that he schemed to sell President-elect Barack Obama's vacant Senate seat for campaign cash or a plum job for himself. He was also accused of trying to strong-arm the Chicago Tribune into firing editorial writers who criticized him, and pressuring a hospital executive for campaign donations.

Outraged lawmakers appointed a committee to investigate Blagojevich and to issue a recommendation on whether he should be impeached.

Blagojevich attorney Ed Genson had no comment on what restrictions Fitzgerald should place on the committee.

Perhaps previewing his courtroom arguments, Genson yesterday questioned the legality of the wiretaps. He warned the impeachment committee: "I think you're using evidence that was illegally obtained."