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Over lunch in Manhattan several months ago, Brendan Carroll, a partner and co-founder of the private equity firm Victory Park Capital, had a proposition for his former boss, Joseph I. Lieberman, the former senator from Connecticut and onetime vice-presidential candidate: Would Mr. Lieberman be interested in joining his firm?

Mr. Lieberman, who retired from the Senate last January after representing Connecticut in Washington for 24 years, was intrigued and eventually decided to join Victory Park as chairman of its executive board, the firm announced on Thursday.

“While in the U.S. Senate, I fought for policies that would allow small businesses to thrive,” Mr. Lieberman said in a statement. “I look forward to a long-term partnership with Victory Park Capital that will position the firm for continued growth.”

Mr. Lieberman’s appointment comes after his move to the law firm Kasowitz Benson Torres & Friedman in New York last summer.

Before being elected to the Senate in 1988, Mr. Lieberman, 71, served as attorney general in Connecticut for six years and as a state senator for 10 years before that.

Mr. Carroll, 36, worked as a staff assistant in Mr. Lieberman’s office while studying American government at Georgetown University in the mid-1990s. The two men kept in touch, and Mr. Carroll said he took a leave of absence from his job at the investment bank Robertson Stephens to join Mr. Lieberman’s 2000 vice presidential campaign.

“Obviously, the personal relationship helped,” Mr. Carroll said. “But he did a lot of his own work to determine that this was something he wanted to do.”

Victory Park invests in mid-market companies across a range of industries, including oil and gas and airlines, and could benefit from Mr. Lieberman’s extensive regulatory expertise, Mr. Carroll said.

“I’ve been fortunate enough to be able to work for him and see what a great person and knowledgeable person that he is,” Mr. Carroll added.

Politicians have long been attracted to private equity firms, a love affair that has drawn criticism at times. Timothy F. Geithner, the former Treasury secretary, joined Warburg Pincus in November, a move some observers decried as being inherently conflicted. Mitt Romney’s ties to Bain Capital drew sharp criticism during his 2012 presidential bid, when opponents accused the firm of outsourcing American jobs.