Less than four years ago, Representative Jackson was eyeing the Senate seat that Mr. Obama would soon give up to go to the White House. A poll and local editorials backed him, and Mr. Jackson told everyone who would listen why he should get the seat, distributing talking points and templates for letters of support and, on occasion, carrying a three-ring binder that made his case.

The Senate was only one option. By then, he was regularly mentioned as a future mayor of Chicago — one of the few figures thought of as legitimate competition to Richard M. Daley, a fellow Democrat who ran the city for decades, his father before him. There was no rush, though, the thinking went: Mr. Jackson was young and had plenty of time…

The events — including a continuing House ethics investigation into claims that a longtime supporter and friend offered sizable campaign contributions to Rod R. Blagojevich, then the governor, if he would appoint Mr. Jackson to the Senate seat being vacated by Mr. Obama — have called into question the Jackson family narrative that some Chicagoans had come to believe: that the workhorse son with the privileged education and crossover appeal might surpass the prominence of the father, who twice ran for president but was best known for mobilizing opinion outside establishment circles.

“He’s frozen,” Don Rose, a longtime Chicago political consultant, said of the younger Mr. Jackson, who is still widely referred to here as Junior. “He had a high potential. But the ability to go beyond Congress — to be mayor, to be senator — is pretty well dead, unless something changes and the clouds are removed.”