"Mr. President, when you are wrong, I will criticize you," Parren Mitchell told Jimmy Carter one recent morning at the White House. The President looked around at the grim faces of the all-Democratic Congressional Black Caucus, nodded to Mitchell on his right, and replied, smiling, "I know."

Outside, the press corps siezed Mitchell, chairman of the 16-member caucus, who has unhesistantly aimed his own verbal arrows of "neglect" at the White House. "Were you promised a domestic policy,?" they asked. "What about full employments, what about jobs?"

"No, we weren't," said Mitchell, looking past the notepads and news clippings of polls indicating of waning confidence of blacks in the administration they earlier had put great faith in.

"But we are promised a hard look," said Mitchell, adding, in a deliberately wry tone, "and we have to assume that the President wouldn't lie back to the Black Caucus."

On occassion Parren Mitchell, a Maryland congressman since 1970, is the guarded politician, stepping gently but still jabbing. A more frequently heard Mitchell is the unhappy warrior, the man who for months has claimed that the nation is entering a "Second Reconstruction," as he sees the hard-won gains of the civil rights era chipped away.

"Well, black folks are going down the drain again," said Mitchell, whirling into his Capitol Hill office and thumping a Pall Mall on his desk. Though he spoke calmly, he looked worn, his pegged face smoldered with frustration. It's not only a political viewpoint, a subject for fiery rhetoricon the speaking circuit but a personal setback, because that century-long fight for social equality has motivated Mitchell, a member of the prominent civil-rights family, from the cradle to Congress.

And, curiously, while many have seen a worsening national picture for minorities, the star of Parren Mitchell has steadily risen.

In other years, Mitchell grabbed the headlines only for interceding dramatically in prison disturbances, and last year, for running up an overdue phone bill. Now he's becoming increasingly influential as he leads discussions with Carter and Cabinet Officers.

Today and Saturday the Black Caucus is holding its annual two-day summit of political and economic workshops and social events. Mitchell will be the host to Jimmy and Rosalynn Carter and 3,000 other guests at the group's fun-raiser Saturday evening.He will make his first appearance on "Meet The Press" the next morning.

Yet he's snarling. "Yes, I'm pessimistic. There's a new mood in American that's anti-black. its painful," said Mitchell, his words, bouncing off the photographs of those milestones in black progress, the signing of the civil-rights bills of the '60s.

Ironically, these warnings came as the new Democratic administration settles in. After his initial meeting (shortly after the Inauguration) with Carter, the first Democratic President the Black Caucus has had to talk to since its formation in 1970. Mitchell said, "We are not losing any ground." After a second meeting Aug. 31, he askws, " Has the status of black folks substantially changed since the President got into office, and answered himself, "No."

"Each of our meeting has been cordial and pleasant," said Mitchell. "He has responded to the criticism of his programs by blacks. I would like to believe that the President is a decent human being. The question is, is his game plan more important than the immediate issues? In a way, I understand the long-range view. He's a technician, he's part scientist, and that's the way their mind work."

He turned to the telephone, excused himself, and placed a call to the White House. He's the practical politician, keeping the lines open, especially when you have had to use the back door for so long. Sharing the Spotlight

"All right, let's huddle," Mitchell ordered his caucus colleagues before a press conference. They huddled. And when Mitchell finished discussing the Bukke Case on reverse discrimination for the media, all the caucus members sat down. That's Parren Mitchell, 55 years old, a trim, 5 feet 5, who's nicknamed "The Little General" because he's quick on his feet, organizes his forces well and barks out at his own impatience with trivia.

He's not a limelight grabber.

Cardiss Collins, a caucus member from Chicago, explaining. "Parren works hard to make sure each person gets his share of spotlight. He's forging the caucus forward because now we are acting rather than reacting."

In looking at Parren Mitchell, a man who has not only been a participant but a test in Maryland civil-rights activities, who has been criticized as a superficial renegade but now is generally respected as a bright and hard working legislator, there are many sides, more paradoxes.

Well-known is the outspoken crusader, part of a leadership reserve - the Mitchell and Jackson families - active for generations in Baltimore. Parren's older brother, Clarence, chief lobbyist for the NAACP, is a respected force in the Capitol Hill; one nephew is a Maryland state senator, another is a member of the Baltimore City Council.

Also, there's Mitchell the recipient of the Purple Heart in the World War II, who is now a pacifist and votes against all military aide, a position his Jewish constituency loathed in his early terms. Also, there is the sometimes-acid orator who lovingly cares for the roses and grapevines in his backyard. And there's the Episcopalian who lays a reader in his church but receives "more of a religious experience," according to a friend, from hearing black kids sing spirituals or attending a senior citizens' play.

He's the liberal colleague who helped removed Rep. Robert Sikes (D-Fla.) from a powerful subcommittee chairmanship on a conflict of interest question and the liberal voice who called very early for Richard Nixon's resignation and who sued Nixon over the bombing in Cambodia - yet who admires John H. Rousselot (R-Calif.), former national publicity director of the John Birch Society, for his grasp of details. The staffs of their two mutual committees call Mitchell and Rousselot "The Bookends." Said Rousselot, "I certainly would call Mitchell a strong advocate of his view. He is more active than a lot of other members of the committee. We do differ but we get on fine."

Timidness? Not before presidents, but a former female staff member commented, "Parren is essentially very shy. He will walk up to you and say, 'Hey, baby doll' or 'Devil,' that's his other pet name, but if you tease him like that, he gets embarrassed, stammers and walks away."

Only one group has drawn his rancor: the drug pushers. "Essentially I'm a liberal but I can't take a public stand on the de-criminalization of marijuana," he explained.

"As a probation officer I saw guys coming off drugs cold turkey. They would be moving in their own feces, their faces would be all bloody from banging their heads against the wall. These people, the drug traffickers, cause me to be less than a liberal."

Over the last 18 years Walter Orlinsky, the president of the Baltimore City Council and a candidate for governor, has worked with and watched Parren Mitchell. Mitchell has changed, in Orlinsky's view, from an intergrationist to a man who only saw the black side of issues, to a bridge builder.

"I wouldn't go as far as to say a concialator. But the Parren Mitchell who has less of a tendency to get involved on the black side of the black-and-white issue is a better person for the city. The only time he strays is when he gets too angry on a peripheral issue." Seething Anger

Yet, his anger, a stinging backhand to any form of injustice, keeps Parren Mitchell Keep on keepin' on .

His anger can erupt. When then Gov. Spiro Agnew called together a group of black leaders after riots in several Maryland cities, and then berated them, Mitchell stormed out. Another time he exchanged sharp words with then-Secretary of Housing and Urban Development George Rommey at a 1971 meeting with Richard Nixon that the Black Caucus had waited 14 months to have.

His anger can seethe. After two nights watching "Roots," Mitchell turned off the series and said publicly, "If I had met any of my white friends I would have lashed out at them from a vortex of primeval anger." Said his sister-in-law, Juanita Mitchell, an activist attorney, "Parren's emotion is the emotion of a Patrick Henry. He didn't say give me liberty later.Parren is just one of God's angry men."

"I know I get emotional. But it's been more than 20 years fron 1954 (the year of the Supreme Court decision on the school desegration, the beginnings of the modern civil-rights movement). How many people have been jailed, lost, and now you see those gains unraveling," said Mitchell.

"When I get angry I take 30 minutes by myself to think, quietly, just thinking. I'm not ashamed of my anger, I had a very close friend, Walter Carter, who had taken his beatings, tried to changed the laws. We were ready to work on the next level, forming the laws from inside the system. Then one night Walter died, just died at a public meeting. I never have gotten over it, that's why I'm angry, so that lives like Walter's wouldn't be wasted." Family Voices

So, in many ways, Parren Mitchell grew up enraged, hearing the fatigued but never hopeless voices of his family plan the next boycott. Elsir and Clarence Mitchell gave their sons time for the boyhood games of "King of the Mountain," but also gave them a picket sign.

With his older brother, Clarence, Parren Mitchell marched outside the department stores, the amusement-park gates, the theater box office. Now many of those businessmen are Mitchell's constituents in a district that includes part of downtown Baltimore, Coppin State College, mid-income neighborhoods and housing projects.

As a youngster Mitchell worked as an elevator operator and delivery boy, graduated from Douglas High School in 1940, served in Italy during the war and then earned a degree in sociology from Morgan State College.

He had to sue to enroll at the University of Maryland graduate school in 1952. "You never did things because you were a Mitchell, there wasn't that kind of pressure, you did things because it was right," said Mitchell. for the next few years, Mitchell worked as a probation officer, a supervisor of the city's probation department, as executive director of the State Human Relations Commission, and then he headed the Baltimore anti-poverty program.

"The experience showed Parren's foresightedness," says the Rev. Vernon Dobson of BAltimore, a friend. "He knew the program wouldn't last, so he insisted that upward mobility would be more than a sociological term. He made sure that the people not only get a job with the agency but also continue their education. He wanted the people to have something when the program is over."

In the late '60s Mitchell turned to teaching, at Morgan's Urban Studies Institute, and to elected politics. In 1968 he organized a primary drive in six weeks and lost by less than 5,000 votes to an 18-year incumbent. Mitchell won the next election, a pattern that hasn't been broken yet.

"One of P's biggest frustration when he ran the anti-poverty agency," said James Joseph Howard, an associate judge on the Supreme Bench of Baltimore, "was that he knew what needed to be done but he would be hamstrung by others making the decisions, others designing the budgets. He wanted power, the power to change, and he has a little more now, but he still hasn't slowed down." Back in Baltimore

Each evening, except in rare, emergency circumstances. Mitchell and a stall aide get in his 1976 Granada and drive back to Baltimore where he lives. Often the congressman hops out a few blocks away from his house, stopping in the grocery stores and the neighborhood bars, checking on happenings.

When he gets to his own modest three-story brick rowhouse, distinguished only be a spiral staircase in the dining room. Mitchell, who is a bachelor, cooks, tends to his house plants and talks to his constituents on the phone. "He subjects himself to his constituency in a manner that's almost unfair to him," said one staff member, Clarence Bishop.

One Saturday night, when Mitchell had several appearances scheduled, the phone rang and a young girl told him it was her mother's birthday. She said her mother idolized Mitchell and could the congressman stop by her birthday party. Mitchell said it was impossible, but he would talk to her mother on the phone. The girl hung up. A few minutes later her brother called, apologised for the request and told Mitchell he knew the congressman didn't have time for birthday parties. Before the evening was over, Mitchell surprised the family by showing up.

Once Mitchell toyed with the idea of running for mayor of Baltimore but now thinks Congress fits his game plan. "It is clear that this is where the damage is going to be," he says.