When Henry Marsh, Richmond's first black mayor, wanted to stop the state of Virginia from building an interstate beltway around his city, he went directly to the White House.

The result: a strong letter from Transportation Secretary Brock Adams to Gov. John N. Dalton warning that the $283 million project might not get essential federal funding unless the state reached an accommodation with Marsh.

Officials at Richmond's City Hall, downtown Richmond's fragile retail economy, were jubilant. At the State Capitol two blocks away, some officials muttered privately about "political blackmail."

The Adams letter illustrates how Democrat Marsh, in his first full term as mayor of Richmond, has used his Washington connections to gain leverage over a hostile Republican state government and carve a larger role for himself in a city where the mayor's job traditionally has been that of a ribbon cutter.

It also suggests that President Carter's urban black constituency, which was crucial to his 1976 election victory and to his 1980 hopes, may not be deteriorating as rapidly as some pollsters and politicians believe.

At first glance, Henry Marsh may not appear to be an important national political figure. The 45-year-old lawyer is a soft-spoken moderate with little of the flair of Atlanta's Maynard Jackson or Washington's Marion Barry.

But behind the scenes, Marsh has built a formidable reputation with groups like the NAACP and the National League of Cities. While Barry was marching in blue jeans and dashiki for civil rights in the 1960s, Marsh was working for the same cause in a three-piece suit in Virginia coutrooms. He and his law firm did much of the work free, and he became a familiar figure with influential NAACP leaders such as Patricia Harris, now secretary of Housing and Urban Development.

"Obviously it helps to know the people making the decisions in Washington and I know quite a few of them" says Marsh, who last month gave a keynote speech at the NAACP's Washington celebration of the anniversary of the -rown decision, and this month is touring China with a dozen fellow mayors.

It also didn't hurt that Marsh was an early and ardent Jimmy Carter supporter in 1976 and has continued to be one of the president's strongest urban boosters. And as the first black mayor of the former capital of the Confederacy - a city that for decades has been controlled by wealthy white businessmen - Marsh has become a symbolic figure of new black rule in the cities.

"He's clearly a politician who is perceived as having influence with a national black constitutency," says Tim Smith, who directed Carter's 1976 Virginia campaign and is legal counsel for the president's reelection committee.

Marsh took office after an unprecedented 5-to-4 black majority was elected to the City Council in 1977, a milestone in the slow but steady political rise of Richmond's blacks.

Although blacks account for 42 percent of Richmond's 215,000 residents, the city's white power structure long has resisted their political influence. In 1970 city leaders engineered a controversial annexation that diluted the black population from a high of 52 percent by the addition of 23 square miles of largely white suburbs to Richmond.

Blacks fought the annexation in the courts and ultimately lost. But in the process they won a ward system that helped Marsh become mayor.

The battle was a bitter one but was resolved, as many Richmond civil rights issues have been, without open conflict. "This community was always more interested in the ballot box than in taking to the streets," says Harold Marsh, a Richmond lawyer and the mayor's younger brother.

Under Marsh the changes in Richmond's government have b-en subtle, reflecting his low-key style.

"Henry plays it close to the vest," says Councilman Wayland Rennie. "He's a strategist and he's no fool. He knows who his friends are and who his enemies are."

There is more emphasis on neighborhoods and housing in the new Richmond, and more aggressive pursuit of federal dollars.But the fears of many whites that Richmond's tax rate would climb proved groundless, and the city's prized AA bond rating has endured.

The most acrimonious dispute was last year's firing of William Leidinger, a white city manager with whom the council majority clashed. The council replaced him with Manuel Deese, who is black. Both Deese and Leidinger learned urban administration while assistant city managers in Alexandria, and City Hall workers say their styles are remarkably similar.

Still the ouster incensed white business leaders who were quick to see its racial overtones. "It reawoke their worst fears," says Rennie, a member of the white minority.

With Leidinger gone, observers say Marsh has been able to exert more influence over Richmond's government, which, unlike the District's, is administered by the city manager, not the mayor. Marsh's role is growing partly because the new city manager is more amenable and because Marsh wields almost total control over the council.

"He makes sure they [the black majority] close ranks on every important issue," complains former vice mayor Henry Valentine II, a downtown stockbroker. "They have done to us exactly what they said we used to do to them.

Despite some uneasiness, white leaders have tried to make peace with Marsh. "He's a good listner and he keeps the lines of communication open," says banker Carlton Moffatt, president of the Chamber of Commerce.

To symbolize his acceptance, Marsh has been invited to meetings at the whites-only Commonwealth Club, a traditional bastion of Richmond's elite. Before he became mayor, Marsh had never been there.

A Richmond native, Marsh lives in one of the city's oldest black neighborhoods. He usually drives his three children to school and is downtown by 8 a.m. It's not unusual for him to return home after midnight.

By turning the somewhat ceremonial job of mayor into a virtual full time position, Marsh now finds himself working two jobs - that of a lawyer and of a strong mayor. Under the Richmond form of government, Marsh could have left the running of the city up to the appointed city manager.

"Henry loves Richmond and he loves politics," says longtime law partner and friend S.W. Tucker. "He loves the art of the possible"

But all of Marsh's political skills haven't made much of a dent in the conservative Dalton administration here. Marsh's urban black constituency is the direct opposite of the governor's white, largely suburban and rural support.

"You could say there's a little creative tension," says Richard DeCair, executive director of the Virginia Municipal League.

Marsh directly lobbies the White House and federal agencies for money for housing, jobs and other programs. In the pipeline at the moment is a $4-million HUD grant the city needs to help renovate the aging Jefferson Hotel, a downtown landmark since the turn of the century.

But while he makes the 100-mile trip to Washington about twice a month, Marsh has met with Dalton, whose offic is a few hundred yards from his, only twice in the last year and a half.

"Obviously, Henry Marsh decided a long time ago that he could get more from Washington than he can get here," says a statehouse source. "He may be right, but it's a risky game."

Marsh's latest confrontation has been over the Richmond beltway, which suburban interests have pushed and Dalton has endorsed. Marsh has insisted that surrounding counties or the state provide revenues to the city to make up for the damage he thinks the beltway would do to the city's tax collections. So far no one in the subburbs has endorsed the idea.

So Marsh went directly to the White House where he met with presidential aide Jack Watson. Watson says he agreed with Marsh that the roadway violated the president's urban policy of avoiding projects that could do economic damage to cities. Watson says he consulted with Brock Adams inspiring Adams' letter, which arrived here just as the State Highway Commission was considering the project.

"Brock made it clear that nothing will go forward until Henry signs off," says Watson assistant Bruce Kirschenbaum.

Infuriated over what it deemed federal interference, the state commission approved the beltway anyway. Insiders say a compromise with Marsh, which the state had hoped to avoid, is inevitable.

In return for the political leverage, Marsh has pledged loyalty to Jimmy Carter. "I think he's doing an excellent job," says Marsh.

And if Carter loses in 1980? Marsh says he still intends to exert whatever Washington influence he can muster.

"We live in a political world and politics are important," he says. "In a sense, we're in competition with other cities. If we don't have strong leadership and make every effort in Washington, the city will be shortchanged." CAPTION: Picture 1, MAYOR HENRY MARSH...makes office full-time job; Picture 2, Mayor Henry Marsh of Richmond poses with city's modern City Hall behind. UPI