Mr. Rogers said he believed that Mr. Scheunemann was hired because he had worked in Congress for more than a decade and had experience with investigations, and not because of any ties he had to Mr. McCain. He added that Mr. Scheunemann had served the firm in an advisory role, and had never spoken with Mr. McCain about the issue.

Image Randy Scheunemann, foreign policy adviser for John McCain. Credit... Mary Altaffer/Associated Press

Since the Russian invasion of Georgia, Mr. Scheunemann has drawn attention for his lobbying efforts on behalf of the Georgian government, for which he lobbied until March. Mr. McCain has been outspoken in his support of Georgia. During a flight on Tuesday on the McCain campaign plane, Mr. Scheunemann told reporters that Mr. McCain has known the Georgian president, Mikheil Saakashvili, for more than a decade.

Craig Holman, the governmental affairs lobbyist for Public Citizen, a consumer advocacy organization, said Mr. Scheunemann’s dual role  sometimes advising Mr. McCain as a candidate, and sometimes advising private clients on their interactions with him as a senator  raised potential red flags. “This is a serious revolving door problem: a person who keeps fluctuating between being a lobbyist, and advising candidates,” Mr. Holman said.

The McCain campaign imposed more rigid rules governing lobbying in the spring, but a number of senior advisers have been lobbyists in the past.

After the Abramoff scandal was brought to light in 2004, Greenberg Traurig ordered Mr. Abramoff from his office, conducted an internal investigation into his conduct, began working with clients he had misled, and cooperated with outside inquiries. A spokeswoman for Greenberg Traurig did not respond to a message seeking comment that was left after office hours on Wednesday evening.