Paul C. Light, a professor of government at New York University who worked closely with Senator Thompson on campaign finance and ethics proposals, said that lobbying by a family member was “an ugly practice no matter who the senator is, because it creates the appearance that his family is exploiting his stature and position.”

Mr. Thompson was hardly the only lawmaker with relatives in similar positions. As money pouring into lobbying firms has soared in recent years, many lawmakers have watched family members get into the business. But ethics experts say the case is all the more striking when the relatives, like Tony and Daniel Thompson, have scant qualifications other than their family ties. And the elder Mr. Thompson was one of the Senate’s most outspoken advocates of tighter ethics and campaign finance rules.

Some groups whose interests and causes the elder Mr. Thompson supported in the Senate also hired his sons. In 1998, for example, Lockheed Martin hired Tony Thompson and his partner, Mr. Mathews, to lobby the federal government to maintain contracts to manage federal nuclear facilities in Oak Ridge, Tenn.

Lockheed had been a major source of money for Mr. Thompson’s Senate campaigns; around the same time he had hired a former Lockheed lobbyist, Powell A. Moore, as his chief of staff. In November that year, Mr. Thompson and Tennessee’s other senator, Bill Frist, invited Bill Richardson, then the energy secretary, for a tour of the facility to make the case for retaining Lockheed.

The next month, Mr. Richardson deferred a decision about the fate of the contracts for 15 months. “Secretary Richardson is to be commended for making a thoughtful decision,” Mr. Thompson said at the time.

Lockheed lost the Oak Ridge contracts in 2000, two years before Mr. Thompson left the Senate. The loss of the contracts, Tony Thompson argued, showed that there was “obviously no connection” to his father.

There were some causes that the senator supported before his son lobbied for them. As an actor, for example, Senator Thompson was a natural champion for the Motion Picture Association of America on subjects like copyright protection and obscenity regulation. As a former trial lawyer, he was sometimes one of the few Republicans to oppose limits on personal injury suits and lawyers fees. And as a former lobbyist for certain cable television concerns, it may have been natural that Mr. Thompson supported the cable television industry’s goals in a regulatory overhaul of the telecommunications industry.