At one point while he was testifying in his trial last week, Rep. Charles C. Diggs Jr. in an aside described himself as a "private person."

Reserved to the point of aloofness, stiff and formal in appearance throughout the 10 days the trial took. Diggs wore more the somber demeanor of a funeral director than that of a politician. And, of course, that's exactly what Diggs had been until 1951, when he was elected to the Michigan State Senate.

He had come back from World War II, after serving what was known then as the "black Air Force" to work in the family business, the House of Diggs funeral home. An only child, he was president of the business. His father, who founded it, was a politician and served in the State Senate until he was indicted and convicted for graft in public office.

So Diggs ran for his father's State Senate seat and won . And four years later he ran for the House of Representative from the 13th District and won - the first black man elected to Congress from Michagan. The funeral home was a kind of natural power base in the community, a place where people came in times of trouble.

Diggs went off to Washington in 1955 and had no trouble winning in every succeeding election. In time, because that is the way in Washington, he rose on the totem pole of seniority.

He was active in civil rights affairs. Several times during the trial Diggs and other witnesses attempted to say that he had raised $10,000 to help support the Montgomery bus boycott, the incident that crystallized the civil rights movement and gave the movement its Moses - Martin Luther King Jr.

Diggs founded the Congressional Black Caucus and served as its first chairman. He became chairman of the House International Relations Sub-committee on Africa at a time when black Americans were rediscovering their roots.

His succession to the chairmanship of the House District Committee provided the opportunity for passage of home rule for the District of Columbia.

The passage of time had made him the senior black member of Congress. "If this were a testimonial dinner, we would all stand and applaud Congressman Diggs for his accomplishments," Justice Department prosecutor John T. Kotelly said in his closing argument to the jury on Friday. "But this is not a testimonial dinner."

Indeed, Diggs, the self-described "private person," also had a private life. His first two marriages ended in divorce, with substantial alimony payments and mortgages for him to pay. Now remarried, he has six children of varying ages to support.

The family business, the House of Diggs, suffered from lack of attention. During his trial he described in painful detail what needed to be done for a black funeral home to collect from its customers. As Diggs' expenses went up, the income from the family business went down.

By 1973, according to Diggs' own testimony, his financial condition "was very bad." He was not meeting his monthly bills, he said. "The creditors were on my back. It was not a good picture, no question about it." Without the help of his staff, especially his office manager and eventually the prosecutor's main witness against him, Jean G. Stultz, Diggs said he would not have been able to do his job. Stultz was a "buffer" between him and the bill collectors.

Eventually, Stultz became something else - a source of money for Diggs. He lived comfortably, by most accounts, but not lavishly. His average income, including congressional pay, income from the House of Diggs and the salary of his third wife, Janet - a Foreign Service officer - was more than $70,000 annaully for the years 1973 and 1976.

But his bill were more. Stulzt alone paid more than $20,000 of them in three years - mortgage payments, flower bills, loans, car repairs, even a subscription to "Moneysworth."

Back in his home district there had been some accusations that the government's prosecution of Diggs was the white establishment's way of knocking him off just as he was tightening his grasp on some power.

Seen from another perspective, the Diggs trial was just another in a long, lamentable line that begins before Tammany Hall and Teapot Dome and is punctuated with similar convictions of public officials through the 1960s and '70s right upuntil yesterday's verdict in the United States District Court for the District of Columbia.