PITY MARION BARRY. The mayor is the most successful and powerful politician in the history of modern government in the District of Columbia -- an intriguing, irrepressible Democratic boss with a tight rein over his political machine and a lock on his office. Yet in the bizarre, paradoxical world of District politics, the mayor is the captive of his own success, a man who made it to the top of the local political heap in record time and now, literally, has no place to go.

The sad, and rarely spoken, truth of decade-old home rule government in the nation's capital is that local elective politics is a dead end, a one-way ticket to obscurity. The more powerful and prominent you become in this town, the greater the frustration at having nowhere else to go.

This dilemma is far more than a personal tragedy for the ambitious mayor and a handful of other aging veterans of the civil rights movement who came to power here in the late 1960s and early 1970s. The bottleneck has stifled the development of a new generation of political leaders moving up the ranks.

"I think there's stagnation," said City Council member John Ray (D-At large), a six-year veteran of the city council and a candidate for mayor in 1982. "Those of us on the city council, we don't do very much to encourage and help bring up some shining stars to take our place. Quite the contrary. We might try to put our foot on the shiny stars."

With few opportunities for advancement, the city's best and brightest have either languished in the political backwaters of advisory neighborhood commissions and local Democratic Party activity or done something totally different. It fact, it can be argued that the city's new political elite is not made up of elected officials as much as aggressive, mostly blackbusinessmen and lawyers who helped elect Barry. By law, at least 35 percent of all goods and services purchased by the city each year must come from minority firms certified by Barry's administration, and they are among those reaping the benefits.

Barry, who will turn 50 next year, probably could stay on indefinitely as the District's chief executive. After a shaky first term, marred by budget crises and a water- billing system that ran amok, he crushed the late Patricia Roberts Harris and other worthy challengers in the 1982 election and solidified his political base citywide. Already there are signs he may have no serious opposition when he runs for a third term in 1986. But what beyond?

"At some point you want to do something else, we all do," Barry said wistfully during a recent interview in the backyard of his two- story home on Suitland Road SE.

But for Barry, a former school board and city council member, there is no lofty U.S. Senate seat beckoning; the District's 623,000 residents lack representation in the Senate. The city's lone seat in the House of Representatives, held since 1971 by D.C. Del. Walter E. Fauntroy, doesn't interest him. Why would Barry give up a $78,630- a-year job overseeing a 33,000-man bureaucracy with a $2 billion-a-year budget to become a congressman who, by law, can vote in committee but not on the floor of the House?

With the Republicans firmly entrenched in the White House, Barry has no immediate prospects for a cushy, high-profile federal appointment. The mayor is a maverick Democrat, more in step with Jesse Jackson than Democratic National Committee Chairman Paul Kirk, and he might not find the pickings much better under a Democratic administration. Even a lucrative sinecure in a K Street law firm is out of the question. Barry made the youthful mistake of studying chemistry instead of law at LeMoyne College.

Barry admits he sometimes dreams of higher public office. He says he would like an appointment to a "non-traditional" cabinet post -- meaning something other than secretary of Housing and Urban Development or Health and Human Services, posts frequently filled by prominent blacks. (Pat Harris held both positions during President Carter's administration.) An appointment as secretary of defense or director of the Office of Management and Budget (OMB) would be nice, Barry said, though not realistic.

Effi Barry would prefer to see her husband appointed a U.S. ambassador, according to Barry, and that would be fine as long as it's not another "traditional" posting to some African nation. "I don't like the traditional boxes they put blacks in," he said.

But if Barry can't move up the political ladder -- which now appears to be the case -- then he wants to make some money. His friends Elijah B. Rogers and Ivanhoe Donaldson parlayed high-level city government posts into high-paying jobs with, respectively, Alexander Grant & Co., a large accounting firm, and E.F. Hutton Inc.

"You know, most of us (in District politics) . . .don't have any wealth," Barry said. "I live from paycheck to paycheck." I've got to earn a living (after leaving office), but I'm not earning it hustling around."

But just which big-name corporation eventually will hire this highly controversial, strong-willed big-city mayor remains to be seen. Barry is haunted by the fate of others who fell on hard times after leaving government. Sterling Tucker, a former City Council chairman and mayoral candidate, had to scratch for city contracts to keep his consulting firm afloat. Willie J. Hardy, a former City Council member, sold wreaths and pillow cases last Christmas to try to make ends meet.

"That's a tragic situation," Barry said.

Former Mayor Walter E. Washington is one of the few political heavyweights to make a relatively graceful transition from public office to private life. Today he is a member of a Washington law firm and sits on a number of corporate boards.

In any political world except the District of Columbia, it would be inconceivable to hear such wistful thoughts about the future coming out of the mouth of a big-city mayor at the top of his game. Granted, it's not easy to parlay a job running a city into higher elected office. John V. Lindsay of New York and Sam Yorty of Los Angeles proved that.

But nowhere else do you hear no talk at all about a successful politician moving on to higher office.

Thomas Bradley of Los Angeles, who came agonizingly close to becoming the first black governor of California, is now discussing a bid for the Senate. Dianne Feinstein of San Francisco was mentioned for the vice presidency. Henry Cisneros of San Antonio is constantly being touted as Texas' or America's next Hispanic you-name-it. William Donald Schaefer of Baltimore is running for governor of Maryland. Federico Pena of Denver and Wilson Goode of Philadelphia are attractive young mayors of whom no one thinks we've heard the last.

In similar circumstance, a move to the Senate or House would be a logical next step for Marion Barry. Barry says he probably would be bored with life on the Hill, but the point is he'll probably never get the chance to find out. The District remains a political stepchild of Congress without full representation in either the Senate or House.

For nearly 100 years, the District was stripped of virtually all voting rights.

Not until 1964 could D.C. residents vote in presidential elections and not until 1971 could they elect a non-voting delegate to the House of Representatives. In 1978, Congress approved a constitutional amendment giving the District two senators and a congressman -- but subject to the constitutionally-prescribed approval of three-fourths of the states. The District has fallen far short of obtaining that approval and the amendment is due to expire next month.

In states with full voting rights and mature systems of elective offices and political patronage, young, ambitious politicians have room to maneuver and grow and the potential exists for party discipline. Most states have hundreds of elected jobs to shoot for, including seats on city councils and school boards, county and metropolitan boards and commissions, the state legislature, the governorship and the U.S. House and Senate. In many states, the treasurer, auditor, attorney general and county prosecutors also are elected posts.

The District, by comparison, is a political pressure cooker with limited prospects for advancement. Here, there are only 26 elected posts that count for anything: the office of mayor, 13 city council seats, 11 school board seats and the D.C. delegate. Although there has been significant turnover on the school board within the past five years, the more powerful and prestigious city council has remained a fairly static body. Five of 13 council members -- Chairman David Clarke, Polly Shackleton, John Wilson, Nadine P. Winter and William R. Spaulding -- belonged to the first modern-day elected council that took office in January 1975.

The city's network of 323 advisory neighborhood commissioners was envisioned as a potential breeding ground for future political leaders. But apart from the influence they occasionally exert in controversies over zoning changes and liquor licenses, the ANCs generally operates in obscurity. Sometimes it's been hard to find people to run for open seats. Mark Plotkin, an enthusiastic ANC commissioner from the Glover Park-Foxhall Village area of Ward 3, concedes that the system is the "Rodney Dangerfield of D.C. government."

End runs don't work, either. Elsewhere, politicians have won election as mayor after serving in Congress or some high-level federal post. Edward Koch of New York, Harold Washington of Chicago, Andrew Young of Atlanta and Donald Fraser of Minneapolis all were able to make a virtue of their national experience in running for mayor.

Here, the notion of "returning home" to run for a municipal post is viewed with either bewilderment or suspicion. When Pat Harris, the former cabinet member and ambassador to Luxembourg, entered the 1982 Democratic mayoral campaign, she was labled by her critics as an interloper or carpetbagger. Ironically, she had lived here much longer than Barry, who is a native of Misssippi. She served for years as the Democratic national committeewoman from the District.

The limited political horizons in this predominantly black, Democratic city has had the effect of warping or stunting the development of a traditional Democratic organization. There is little point in pretending there is any real political organization in this town other than Barry's own formidable political network.

The Democratic Party does have a few bright lights. Sharon Pratt Dixon, a Pepco executive and the District's Democratic national committeewoman, won election last January as treasurer of the Democratic National Committee. But locally, the party is largely a rule-making and debating society that exerts little, if any, influence over the outcome of city elections.

The system for the most part has beaten down or discouraged young newcomers, a trend that has disturbing long-term implications for the city. As the current leadership ages and runs out of fresh ideas, there is no discernible crop of young political activists coming up to take their place.

Even the small handful of young activists who hitched their stars to established figures in hopes of eventually winning office themselves have had no luck. Notable examples: Johnny Barnes, a long-time aide to Fauntroy who lost two bids for city council seats, and Barry Campbell, a city employe and one-time aide to former council member Arrington L. Dixon, who lost in a bid for the Ward 4 council seat.

Matthew Watson, a politically savvy lawyer and former D.C. auditor, says that in the tiny fishbowl of District politics, there are pitifully few opportunities for up-and-comers to prove themselves before running for the city's highest offices.

"There hasn't been the opportunity to test people along the way to determine where their strengths and weaknesses are," Watson said. "In a normal jurisdiction, people would work their way up to running for city council from the precinct level . . . . We haven't had the gestation period for people to run for office."

Others with obvious potential don't even try.

Ibrahim Mumin, 37, a Shaw-area community activist and developer, decided early on that politics wasn't the route to travel in the District. In 1980, Mumin became executive director of the Shaw Project Area Committee, an agency funded by the city to plan and put together deals to revive one of Washington's most blighted sections.

He also helped form a new community- based corporation that has teamed up with developer Jeffrey N. Cohen, a cloe friend of Barry's, to build a commercial center and low-cost housing in the Shaw area, near 13th and V streets NW. The redevelopment project hinges on a $12.5 million land transaction underwritten earlier this year by the city government.

Mumin is ambitious, articulate, physically attractive and media savvy. In any other city his credentials would have propelled him into local politics and many have encouraged him to run for City Council. Instead, he operates on the fringes of politics, taking advantage of government programs and financing to advance his own social agenda. And he probably will make a lot of money if the redevelopment project works out.

Mumin contends that the strictures of District politics have fostered a cynicism and disillusionment among many young blacks he knows who were attracted to the glitter of politics in the nation's capital, only to find it unattainable.

"For a lot of them it's a status thing," said Mumin. "The money and responsibility associated with other jobs don't have the attraction of (politics) . . . . All you have is the mayor, the council and school board. It seems the rest of the crowd is shooting after them."

Barry recalls a recent conversation he had with two young members of the ANC system who complained that they were growing impatient with politics.

"They said to me, 'What do we do? There doesn't seem to be much room on the council . . . . We stay active in the church, but that doesn't give you any political activity,'" Barry said. "So it's frustrating for them, I'm sure. Where do you go? There's not room. The top is so tight. Eventually, it's going to show itself, too. It's going to lower the caliber of leadership."