“Because of the pendency of the federal complaint, the executive decisions that Mr. Blagojevich makes are tainted,” Ms. Madigan said in her filing with the Illinois Supreme Court. “Indeed, every day this state is faced with an allegedly corrupt governor making critical decisions that have no legitimacy.”

Standard & Poor’s put the state’s general obligation bonds on credit watch with negative implications this week, citing “our concern that the legal charges now facing the governor and his chief of staff may challenge the state to respond to this fiscal situation on a timely basis.”

On Thursday, Mr. Franks filed a measure that would prohibit the governor from temporarily appointing someone to fill a vacant Senate seat “if the governor has been arrested, charged, or indicted for a felony offense” and the case remains unresolved.

The bill also provides that in such an instance the vacancy would be filled in the next election.

“It’s nothing anyone really anticipated,” Mr. Franks said. “I never thought we would need a law like this. But we have to protect the sanctity of our democracy.”

Federal officials acknowledged that a grand jury was weighing evidence in the case against Mr. Blagojevich, though the timing of any indictment was unclear.

In a rare firsthand account of how Mr. Blagojevich, a two-term Democrat, went about the selection process, an Illinois state senator said in an interview that he had felt pressured to respond to the governor’s interest in him with a quid pro quo agreement and has withdrawn his name because of increasing wariness about the process.

The state senator, Kwame Raoul, who represents the South Side of Chicago, offered few details of his interaction with the governor’s office but said he received a call about a month ago confirming that he was under consideration. Soon afterward, however, Mr. Raoul said he ran head-on into another message: that the governor was looking for a candidate who offered something of tangible value to him.