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At least two other Oldaker, Biden and Belair workers have served on Mr. Biden’s campaign committees as either treasurer or custodian of records, according to Federal Election Commission filings.

“It’s just such a tight little circle of influence,” Bill Buzenberg, executive director of the Center for Public Integrity, said Thursday.

“It’s an association that works extremely well for corporations and lobbyists, but you have to wonder what the public interest is here,” he said.

A big Democratic donor, Mr. Oldaker served as an adviser to President Clinton on bioethics issues.

Formerly a partner at Washington-based Manatt, Phelps and Phillips, another big lobbying and law firm, Mr. Oldaker has donated more than $200,000 mostly to Democrats since 1993, according to the Center for Responsive Politics. He has given $8,600 to Mr. Biden’s Senate and presidential campaigns and $3,000 to Unite Our States.

At least one lobbying client at Oldaker, Biden and Belair has been a big backer of the senator from Delaware.

Employees at the Chicago personal injury firm of Cooney and Conway paid Oldaker, Biden and Belair $220,000 in 2005 and 2006 to lobby on tort reform issues. Its employees have given more than $70,000 to Mr. Biden’s political campaigns. The firm did not return telephone messages Thursday.

Speaking for an article on the support that prominent lawyers were providing to presidential campaigns, John D. Cooney, a partner at the firm, told the Chicago Daily Law Bulletin that he got to know Mr. Biden in 2004 when the Senate Judiciary Committee sought to pass a bill that Mr. Cooney thought would hurt his clients who suffer from asbestos-related injuries.

The law firm is no longer a client of Oldaker, Biden and Belair.

Employees at the University of Delaware, a former lobbying client, have donated more than $20,000 to Mr. Biden’s campaigns.

The Republican National Committee has been hammering away at Hunter Biden’s lobbying for days, but it’s not clear whether the attacks will resound with voters.

“I don’t know that it’s going to have that much of an impact because what’s really going to drive voters are the major issues like Iraq, the economy and foreign policy,” said Robert Olendick, a polling specialist at the University of South Carolina.

“The closer we get to the election, there will be more of a focus on Obama. But if Biden portrays himself as an outsider, then the Biden focus might come in and they’ll try to use that as a contradiction against him.”

Mr. Biden announced Unite Our States in fall 2005, a little more than a year before beginning his own failed run for the White House. He said the political action committee sought to elect candidates “committed to addressing the challenges facing our country by beginning to unite red and blue states, big cities and small towns, and Americans of all walks of life.”

One of the biggest recipients of campaign cash from Mr. Biden’s committee has been the Democratic Senatorial Campaign Committee, which has received $65,000 in donations since 2005, FEC records show.