"The president-elect agrees with (Illinois) Lt. Governor Quinn and many others that under the current circumstances it is difficult for the governor to effectively do his job and serve the people of Illinois," Obama spokesman Robert Gibbs told reporters.

On Tuesday Blagojevich was arrested on federal conspiracy charges and went back to work Wednesday after being released on a 4,500-U.S.-dollar bond.

Prosecutors alleged in their complaint that the governor pressured candidates to replace Obama in the Senate for campaign contributions and other benefits.

Obama's former partner in the Senate, Senator Dick Durbin, also called on Blagojevich to step down immediately.

"Beyond guilt or innocence, the charges against you raise serious questions about your ability to carry out your duties as chief executive of our state," Durbin wrote in a letter sent to Blagojevich.

Durbin also asked the governor not to name a successor to Obama.

"Because of the nature of the charges against you, no matter whom you were to select, that individual would be under a cloud of suspicion. That would not serve our state, our nation, or the United States Senate," Durbin wrote.

Even if Blagojevich named a replacement for Obama, it is unclear whether the Senate would seat the governor's choice.

The Constitution gives the Senate the sole authority to decide who is qualified to serve as a senator.

The Illinois Legislature will begin a special session next Monday to consider legislation that would authorize a special election to choose Obama's successor.

Obama said he supported such a move.

Cindy Davidsmeyer, a spokeswoman for Illinois Senate President Emil Jones, said a House committee was scheduled to consider the bill next Monday afternoon and then the full House would vote afterward.

The Senate could consider the legislation as soon as the next day, Davidsmeyer said.

Obama on Tuesday declined to comment on the arrest, saying, "Like the rest of the people of Illinois I am saddened and sobered by the news that came out of the U.S. attorney's office."

Obama also said he had not contacted Blagojevich about his possible successor, adding, "I was not aware of what was happening."

But Obama adviser David Axelrod told a Chicago television station in November that Obama had spoken to the governor about his successor.

Axelrod corrected himself Tuesday, saying that the president-elect and Blagojevich "did not then or at any time discuss the subject."

Meanwhile, US media is keeping a close watch on the development of the Blagojevich case and its impact on Obama.

The New York Times said, "If the world was roused by the sight from Chicago barely one month ago, hundreds of thousands of people streaming into Grant Park to celebrate the triumph of possibility over tainted history, the arrest of Governor Blagojevich on a darkand drizzly Chicago dawn was quite the opposite image."

The Economist magazine said, "Illinoisans, meanwhile, have been jerked from the hazy bliss that blanketed the state since Obama's election. They have long suffered from the state's penchant for corruption. ... Blagojevich represents a new low."

The Wall Street Journal summed up the state's penchant for corruption: "If convicted, Blagojevich would be the second consecutive Illinois governor to be found guilty of a felony, and the fourth in 35 years. We'd ask if it's something in the water, but that would be unfair to the Chicago River."