Mr. Obama was an adviser to Mr. Blagojevich’s first campaign for governor, in 2002, and endorsed him again in 2006, even though by that time questions had been raised about the possible selling of state jobs. Mr. Obama has also credited one of Mr. Blagojevich’s closest confidants, Antoin Rezko, a businessman who was convicted of corruption charges this year, with helping him get his own start in politics.

Image State Senator Emil Jones Jr., a mentor of Mr. Obama. Credit... Associated Press

Mr. Rezko was among the first to contribute to Mr. Obama’s earliest State Senate race, in 1995, and later became a major fund-raiser for his campaign for the United States Senate. Mr. Rezko was known around Chicago as a collector of politicians, and he did not hesitate to make the most of his high-level contacts. The New York Times reported last year that when he was entertaining Middle Eastern financiers at a Four Seasons hotel in Chicago, he arranged for Mr. Blagojevich and Mr. Obama to drop by, separately and on different occasions, to impress his guests.

Mr. Rezko derived his political influence mainly from his close relationship with Mr. Blagojevich, who relied on him to recommend loyal campaign contributors for state appointments to boards and commissions, according to the complaint unsealed on Tuesday. But as Mr. Rezko’s legal troubles escalated, Illinois politicians who had previously found him useful, including Mr. Obama, disavowed him and started returning his campaign donations.

Mr. Obama’s relationship with Mr. Blagojevich was not much better when he made the decision to call Mr. Jones in September about the stalled ethics bill. For Mr. Obama, the move marked an unusual return to Illinois politics, turf from which he had studiously worked to distance himself throughout the presidential race. At the time, one week before the first presidential debate of the general election campaign, Republicans were trying to tarnish him in the eyes of voters by attempting to link him to Chicago’s history of corrupt politics.

Mr. Obama used leverage that he had seldom employed  publicly, anyway  and strongly urged Mr. Jones to bypass Mr. Blagojevich and approve the ethics bill, banning the so-called pay-for-play system of influence peddling in Illinois. When asked at the time how Mr. Obama had come to be involved, Mr. Jones replied, “He’s a friend.”

When the Illinois Senate passed the measure by 55 to 0 on Sept. 22, with Mr. Jones reversing his position, Mr. Obama praised the move as one creating “a tougher ethics law that will reduce the influence of money over our state’s political process.” Mr. Obama’s intervention deepened a rift between him and Mr. Blagojevich that had been growing for some time.

When Mr. Blagojevich left Congress in 2002, he talked openly about the notion of running for president one day. After he was elected governor, and after Senator John Kerry lost the presidential race in 2004, he began eyeing a potential run in 2008.