Congressional Black Caucus members are embroiled in a bitter internal dispute that some fear threatens their group's effectiveness.

Adding fuel is a $2.5 million lawsuit filed in D.C. Superior Court against the caucus' fund-raising adjunct, the Congressional Black Caucus Foundation, and several members of its board of directors, including three congressmen.

The complaint was filed last month by Algernon (Jay) Cooper, a lawyer and former mayor of Prichard, Ala., who alleges breach of contract and defamation of character after being fired as executive director of the foundation. He was hired last December, then abruptly fired in January without explanation.

His suit names, along with the foundation, seven board members who served on an ad hoc committee that voted to fire Cooper. Named are: Rep. Louis Stokes (D-Ohio); Del. Walter E. Fauntroy (D-D.C.); Rep. William L. Clay (D-Mo.); LeBaron Taylor, vice president of CBS Records, New York; Albert L. Nellum, president of A. L. Nellum Associates, consultants, Washington; attorney Nira H. Long, and Ofield Dukes, publisher of the Washington North Star and president of Ofield Dukes and Associates.

Stokes, president of the 20-member foundation board, declined comment except to say, "It's no different from any other lawsuit involving personnel matters."

But the firing caused a flurry of phone calls and letters among caucus members, complaining about the way the matter was handled.

Rep. Harold E. Ford (D-Tenn.), a Cooper supporter, resigned from the foundation board. Rep. Mervyn M. Dymally (D-Calif.) also submitted a letter of resignation to the board in protest, but later withdrew it.

Rep. Parren J. Mitchell (D-Md.) wrote to all caucus members March 19 urging them to suppress their egos for the good of the caucus.

"I am truly, gravely worried about the future of the caucus . . . ," he said. "Black America, desperate, hurt, exploited, and oppressed, needs a strong black voice in . . . Congress.

". . . I write you only to ask that if that dreadful day shall come when the CBC is no more, how would you answer these questions: Did I make the sacrifices I could have made to preserve the caucus? Did I, because of my own frailties arising from ego, individual aspirations or other personal concerns, facilitate the death of the Congressional Black Caucus?"

The foundation, originally a minor caucus subsidiary begun in 1976 to run an intern program, is now an independent, non-profit, tax-exempt organization with responsibility not only for the intern program, but also for fund raising, legislative analysis, research, and voter education projects. Its role increased significantly Jan. 1 when a change in House ethics rules barred fund raising by all caucuses.

The Cooper controversy started almost immediately after the foundation board of directors voted 8 to 7 last Dec. 16 to hire him as executive director. Five board members were absent.

Caucus sources say there was immediate friction between Cooper and some caucus staff members, including petty battles over files, equipment and locks on the office doors.

Hostilities between Cooper and the staff members escalated when rumors surfaced about his contract. The three-year contract, signed Jan. 5 by Dymally, chairman of the board's personnel committee, provided a starting salary of $57,000, increasing to $60,000 in the second and third years.

It also included generous fringe benefits such as travel privileges for Cooper and his family, credit cards, membership fees for the National Democratic Club and the Capitol Hill Club, the right to earn up to $18,000 a year in outside speaking fees, a $114,000 life insurance policy, and a percentage of the foundation fund-raising proceeds up to a $5,700 maximum the first year.

In late January, the foundation board accepted the ad hoc committee's recommendation to fire Cooper. It will now be up to the court to determine if the contract was binding.

Fauntroy, chairman of the Congressional Black Caucus, said in an interview that he does not believe the animosities will last. All other members have declined comment. Fauntroy, referring to the alternative national budget the caucus plans to present to President Reagan April 26, said, "We're doing our best work right now."