WASHINGTON — Senator Obama’s speech announcing his running mate, Senator Biden, singled out the Delaware senator’s son who is headed for Iraq. Mr. Obama didn’t mention the profession of Mr. Biden’s other son, who lobbied for two drug companies and five universities.

Hunter Biden, 38, described as a lawyer in the biography of his father distributed yesterday by the Obama campaign, lobbied for clients that paid his firm at least $380,000 in the first six months of this year, federal records show.

Both Mr. Obama and Republican presidential rival Senator McCain have said lobbyists may not work on their campaigns. Mr. McCain recently called lobbyists “birds of prey,” and Mr. Obama has refused to accept their contributions. In his speech yesterday in Springfield, Ill., Mr. Obama said he’s running in part to repair “a government that has fallen prey to special interests.”

Hunter Biden’s work for Oldaker, Biden & Belair on behalf of biotech clients Achaogen, Inc., of South San Francisco and Pulmatrix, Inc., of Lexington, Mass., illustrates the quandary posed when legislators have lobbyists within the family.

“It’s always something to watch out for when you have a high-ranking member of Congress or an important senator whose child goes into the influence business,” said Bill Allison, a senior fellow at the Sunlight Foundation, a Washington nonprofit that advocates transparency.

“It’s not necessarily that the father’s going to do favors for the child’s clients, but that every other member knows who his father is and what the connection is and that can carry a lot of influence as well,” Mr. Allison said in an interview.

A spokeswoman for Mr. Biden’s Senate office, Elizabeth Alexander, said the senator follows ethics rules.

“Hunter Biden does not lobby and has not lobbied Senator Biden’s office,” Mr. Alexander said in an e-mail. “Our rules are dictated by the Senate ethics bill, and we follow it by the letter.” Ms. Alexander said she knew of no favors done for Hunter Biden’s clients.

Hunter Biden couldn’t be reached yesterday through Alexander and did not return a message left on his office voice mail. His home phone in Washington is unlisted.

The senator’s Web site describes Hunter Biden as a lawyer.

The Obama-Biden campaign had no comment.

Hunter Biden also sits on the board of Amtrak, the passenger railroad whose federal subsidies Mr. McCain has often criticized. Mr. Biden rides home to Wilmington on Amtrak trains every night after work in the Senate.