Senator Barack Obama was joined by Gov. Rod R. Blagojevich, left, and Mayor Richard M. Daley in Chicago in April 2007.

Senator Barack Obama was joined by Gov. Rod R. Blagojevich, left, and Mayor Richard M. Daley in Chicago in April 2007.

In a sequence of events that neatly captures the contradictions of Barack Obama’s rise through Illinois politics, a phone call he made three months ago to urge passage of a state ethics bill indirectly contributed to the downfall of a fellow Democrat he twice supported, Gov. Rod R. Blagojevich.

Mr. Obama placed the call to his political mentor, Emil Jones Jr., president of the Illinois Senate. Mr. Jones was a critic of the legislation, which sought to curb the influence of money in politics, as was Mr. Blagojevich, who had vetoed it. But after the call from Mr. Obama, the Senate overrode the veto, prompting the governor to press state contractors for campaign contributions before the law’s restrictions could take effect on Jan. 1, prosecutors say.

Tipped off to Mr. Blagojevich’s efforts, federal agents obtained wiretaps for his phones and eventually overheard what they say was scheming by the governor to profit from his appointment of a successor to the United States Senate seat being vacated by President-elect Obama. One official whose name has long been mentioned in Chicago political circles as a potential successor is Mr. Jones, a machine politician who was viewed as a roadblock to ethics reform but is friendly with Mr. Obama.

Beyond the irony of its outcome, Mr. Obama’s unusual decision to inject himself into a statewide issue during the height of his presidential campaign was a reminder that despite his historic ascendancy to the White House, he has never quite escaped the murky and insular world of Illinois politics. It is a world he has long navigated, to the consternation of his critics, by engaging in a kind of realpolitik, Chicago-style, which allowed him to draw strength from his relationships with important players without becoming compromised by their many weaknesses.

By the time Mr. Obama intervened on the ethics measure, his relationship with Mr. Blagojevich, always defined more by political proximity than by personal chemistry, had cooled as the governor became increasingly engulfed in legal troubles. There is nothing in the criminal complaint unsealed Tuesday to indicate that Mr. Obama knew anything about plans to seek money and favors in exchange for his Senate seat; he has never been implicated in any other “pay to play” cases that have emerged from the long-running investigation of the Blagojevich administration.

But like those previous cases, this latest one features political characters who figure in various stages of Mr. Obama’s climb from little-known state senator to presidential candidate, and who have since become politically radioactive because of corruption scandals. Some of those relationships posed a threat to Mr. Obama during the presidential campaign, forcing him to return tens of thousands of dollars in tainted campaign contributions and providing fodder for attack ads by rival candidates.

Though extreme examples, they were emblematic of the path cut by Mr. Obama through Chicago politics, where he became known for making alliances of convenience with personalities that seemed antithetical to his self-image as a progressive reformer. His political roots were in the left-leaning neighborhood of Hyde Park, but at key moments in his career he did not hesitate to form relationships with politicians who were fixtures of the Democratic machine.

State Senator Emil Jones Jr., a mentor of Mr. Obama

State Senator Emil Jones Jr., a mentor of Mr. Obama

When he ran for the United States Senate in 2004, he aggressively courted Mr. Jones, a sewer inspector turned legislator who had clawed his way up through ward politics and was viewed as something of a kingmaker in the Illinois Democratic Party. He also formed a good working relationship with Mayor Richard M. Daley of Chicago, a symbol of establishment politics with whom Mr. Obama had never been close.

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.

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.

It was short-lived. The federal corruption investigation that eventually led to Mr. Rezko’s indictment, and Tuesday’s charges against Mr. Blagojevich, had already begun to taint the governor’s administration. And by 2006, Mr. Obama had eclipsed the governor as a plausible national candidate, dashing his presidential aspirations.

The criminal complaint unsealed Tuesday underscored the acrimony between the two men. Recorded telephone calls showed Mr. Blagojevich being far less than respectful when discussing the president-elect and voicing frustration at his inability to advance beyond the governor’s office.

“If I don’t get what I want and I’m not satisfied with it, then I’ll just take the Senate seat myself,” the governor said, according to the criminal complaint. Later, he said the Senate seat was a “valuable thing — you just don’t give it away for nothing.”

Meanwhile, Mr. Blagojevich was busily trying to shake loose up to $2.5 million in campaign donations, much of it from contributors with business before the state, according to federal prosecutors. The governor’s goal was to bring in the money before the end of the year, the complaint said, “before a new state ethics law goes into effect on Jan. 1, 2009.”