Gerald Cassidy, the driving force behind what was once the most lucrative lobbying shop in Washington, is giving up day-to-day leadership of his firm.

Under a transition plan to be announced Thursday, Cassidy, 73, will become chairman emeritus of Cassidy & Associates effective Jan. 1. Barry Rhoads and Kai Anderson will be promoted to co-chairmen of the firm. News of the changes was first reported by Politico.

“After intensive evaluation of the firm’s strengths and opportunities, candid discussion and careful planning, we are making sure we have the right people, in the right place, for the strongest team that will build the next generation of the firm,” Cassidy said in a statement to be released Thursday. “It’s been no accident that Cassidy & Associates has been a leader for 40 years, and I want to make sure it will continue to be one for 40 more.”

In a 2007 Post series that became an acclaimed book, Robert Kaiser dubbed Cassidy “Citizen K Street.”

As Kaiser wrote:

Cassidy's saga is a variation on the classic American myth: A determined man from nowhere accumulates great wealth and rises to the top. At different moments it evokes Charles Foster Kane, Jay Gatsby or a character from a Horatio Alger tale. Like them, Cassidy is a self-made man who fulfilled many of his most ambitious dreams. But material success has not pacified all of his personal demons. He is tough, temperamental, driven and, according to many around him, rather lonely.

As a young legal aid lawyer, Cassidy worked on Capitol Hill for Sen. George McGovern (D-S.D.). He launched his lobbying business in 1975 and built it around helping clients, especially universities, win earmarks.

The firm was long the top revenue generator among K Street firms. But it has fallen steadily over the past decade, and this year ranks 15th among Washington lobbying firms, reporting $9.2 million in revenue through the first three quarters of the year.