CHICAGO, Illinois (CNN) -- Key Illinois Democratic legislators are circulating a letter urging support for the impeachment of Gov. Rod Blagojevich, who was arrested this week on federal corruption charges relating in part to the selection of President-elect Barack Obama's successor as a U.S. senator.

Barack Obama says he has never spoken with Illinois Gov. Rod Blagojevich about the vacant Senate seat.

State Rep. John Fritchey, head of the House Civil Judiciary Committee, sent the letter Thursday asking Democratic colleagues to say by Friday whether they support a move for impeachment and would like to be added as a co-sponsor of legislation. Blagojevich also is a Democrat.

"Faced with a significant budget shortfall, a national recession, and a vacant United States Senate seat, we cannot afford to allow Illinois to operate without effective leadership in the [Illinois] executive branch," said the letter, also signed by Reps. Thomas Holbrook, David Miller and James Brosnahan.

"Simply put, it is imperative to replace Gov. Blagojevich as soon as is practicable."

The letter said the impeachment filing was being prepared, and the lawmakers said they expected the process -- the state House would bring charges against Blagojevich and he would be tried by the state Senate -- would take "a matter of weeks rather than months."

The lawmakers' move coincided with increasing calls for Blagojevich's resignation.

President-elect Barack Obama called again Thursday for Blagojevich to step down, saying the embattled governor can no longer effectively serve the people of Illinois.  Watch as Obama says Senate seat belongs to the people »

"I hope that the governor himself comes to the conclusion that he can no longer effectively serve and that he does resign," Obama said, speaking before announcing his pick to lead the Department of Health and Human Services, former Senate Majority Leader Tom Daschle.

Obama said he was as "appalled and disappointed as anyone" by the allegations against the Democratic governor, and said he was confident his staff was not involved in the alleged scandal.

Federal officials said Blagojevich was looking to sell or trade Obama's open seat in the U.S. Senate.

Obama said he had never spoken to the governor on the subject, adding he was confident that "no representatives of mine would have any part of any deals related to this seat."

He had asked his staff to gather the facts of any contacts with the governor's office about the vacancy, he said.

"This Senate seat does not belong to any politician to trade. It belongs to the people of Illinois, and they deserve the best possible representation," he said. "They also deserve to know that any vacancy will be filled in an appropriate way so that whoever is sent to Washington is going to be fighting for the people of Illinois."

Earlier Thursday, Illinois Attorney General Lisa Madigan said she is prepared to go to the Illinois Supreme Court to have Blagojevich declared unfit to serve if he does not resign.  Explainer: Federal complaint against Blagojevich »

"Obviously the easiest way for us to move on in the state of Illinois is for Gov. Blagojevich to do the right thing for the people and to resign," she told CNN's "American Morning."

She added, "If he fails to, the two other options are obviously the Legislature moving forward on impeachment, or I have the opportunity to actually go to our Illinois Supreme Court and ask them to declare, basically, that our governor is unable to serve," she said.

In that case, Lt. Gov. Pat Quinn, a Democrat, would become acting governor, she said.

Madigan said she won't wait long to take action.

"We would like a signal from the Legislature if they're going to move forward on impeachment proceedings. ... I think there are obviously numerous members of the Legislature calling for impeachment proceedings," she said, noting that the Legislature, which is adjourned, will meet Monday to discuss the possibility of holding a special election for Obama's successor.

Fritchey, in the letter to Democratic lawmakers, commended Madigan's willingness
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