Mark Penn is the chief strategist for Hillary Rodham Clinton’s presidential campaign, working 24/7 on her behalf (indeed, he is known to send email at 2 a.m.) and earning tens of thousands of dollars in fees.

With that sort of day-and-night job, you would think Mr. Penn would have time for little else. Indeed, other Clinton senior advisers took leaves of absences or turned down clients in order to work for the campaign.

But he somehow manages to remain president of Burson-Marsteller, a publicly traded global public affairs firm. And as such, he is inevitably linked to Burson’s clients — not all of whom are, shall we say, on Mrs. Clinton’s Christmas card list.

Today we learned that recently one of those clients was Blackwater — the American contractor that provides security to some top American officials in Iraq, and which is under investigation for allegedly aggressive tactics and the role of its guards in the shooting at a Baghdad city square where 17 Iraqis were killed.

Mr. Penn was on a plane this afternoon and could not be reached for him; a phone message has been left for a Burson spokesman as well. But Mr. Penn and the spokesman told other news outlets today that Blackwater was only briefly a client in a temporary assignment for a Burson subsidiary, BKSH. That assignment involved preparing Blackwater for a recent hearing before Congress. Blackwater is no longer a client, said Mr. Penn, who added that he never worked on the piece of business himself.

“Through a personal relationship, BKSH, a subsidiary of Burson-Marsteller, helped Blackwater prepare for their recent hearing before Congress. With the hearing over, BKSH’s temporary engagement has ended,” said the Burson spokesman, Paul Cordasco.

A blog for the Nation and the Associated Press carried stories on the Burson-Blackwater connection first.

Howard Wolfson, a spokesman for the Clinton campaign, said that Mrs. Clinton had not asked Mr. Penn for an explanation about Blackwater — about which she has expressed concerns on the campaign trail — nor asked him to take a leave from Burson.

“Mark is an extremely valuable member of this team,” Mr. Wolfson said. “He is our senior strategist. Senator Clinton clearly believes Blackwater must be held accountable for its actions.”

About Mr. Penn’s ongoing ties to Burson, Mr. Wolfson said. “His situation is very typical for presidential campaigns. There is nothing unusual about a consultant maintaining outside affiliations. Hillary has trust in Mark.”

Other Democratic and Republican candidates do have consultants whose firms have a bevy of clients as well; Harrison Hickman, the pollster for former Senator John Edwards, one of Mrs. Clinton’s rivals, continues to be a member of Global Strategy Group, which represents insurance and drug companies, for instance.

But Mr. Penn’s firm has netted the sort of bad headlines that Mrs. Clinton would not seek: Other than Blackwater, clients have included Countrywide Financial, which has been tainted by the subprime mortgage scandal, and Cintas Corp., which has fought the unionization of its workforce.

One of Mrs. Clinton’s rivals, former Senator John Edwards, went on the attack this afternoon, meanwhile. His campaign e-mailed a statement from Mr. Edwards:

“Bush has been a perfect example of cronyism because Blackwater has given hundreds of thousands of dollars to republicans and to President Bush. I also saw this morning that Senator Clinton’s primary adviser, Mark Penn who is like her Karl Rove — his firm is representing Blackwater. I think it is important for Iowa caucus goers to understand the choices they have in this election. And it is the reason I continue to say we dont want to replace a group of corporate Republicans with a group of corporate Democrats. I think it is important for caucus goers to see this choice.”

Asked to comment on Mr. Edwards’ shot, Mr. Wolfson repeated that Mr. Penn was a valuable and trusted member of the Clinton campaign.

Mr. Edwards himself was a consultant in 2005 and 2006 with a hedge fund, the Fortress Investment Group, which has had investments in subprime mortgage companies that have foreclosed on victims of Hurricane Katrina. Mr. Edwards has also had a portfolio with Fortress, but has divested holdings that related to the mortgage companies.