After more than three years of trench warfare between white and black politicians for control of Chicago's patronage-rich government, a powerful new chief executive has emerged to run City Hall -- Mayor Harold Washington.

By the slim reed of one vote -- his own -- Washington now controls the fractious, 50-member City Council, ramming through long-stalled key appointments and pressing his entrenched white opponents on every front.

Once held at bay by an opposition council majority pledged to thwart his efforts at change, the black former South Side legislator has said goodbye to political frustration. Improved political fortunes have affected Harold Washington. Blustery rhetoric has given way to the quieter, more confident style of a leader who wields his one-vote majority with the vigor and aplomb of a drillmaster at last free to call the tune for his troops.

"He got a lot of on-the-job training during the first three years," said Alderman Wilson Frost, a staunch mayoral ally. "Now that his time has come, he's ready."

Almost daily through this hot, humid summer, Washington makes headlines, seizing initiatives and turf from the ethnic whites who preside over what remains of the Democratic machine built by the late Mayor Richard J. Daley.

Since May, the mayor has ousted opponents from council committee chairmanships, reorganized council responsibilities to deny power to his foes and appointed more than two dozen new members to the semi-autonomous city boards and commissions that control thousands of patronage jobs.

He has tightened his grip on city minority contract practices, and his senior staff has shown new sophistication in dealing with opposition tactics. "He's more decisive, more in charge," said Marion K. Volini, a lakefront alderwoman who generally backs the mayor. "He's not going to get involved in the political squabbling anymore."

Washington's new clout was made possible by victories of candidates he backed in special elections in four of seven redrawn inner city wards in April and May. A federal court ordered the redistricting and special elections to increase black and Hispanic voting strength.

His gain of four seats cut a 29-to-21 white ethnic council majority to a 25-to-25 tie, with the mayor holding a tie-breaker vote. He seems to delight in his new role.

The principal loser is Alderman Edward R. Vrdolyak, white leader of the anti-Washington faction and a power as chairman of the Cook County Democratic Central Committee.

Vrdolyak denounces Washington for strong-arm tactics and has challenged in court much of what the mayor has done. But local judges, reluctant to intervene, are handing Vrdolyak defeat after defeat.

In recent weeks, the struggle has centered on the mayor's appointment of two board members to the powerful Chicago Park District, which runs 500 parks and is a political fiefdom with a $320 million annual budget and nearly 6,000 employes. It was long ruled by Edmund Kelly, appointed park superintendent by Daley in 1973. A hard-nosed machine loyalist, Kelly saw to it that three out of every four jobs were patronage appointments, giving him plenty of clout.

The district's notoriety as a bastion for whites was underlined several years ago when it agreed to a consent decree in U.S. District Court to equalize resources for parks in black and Hispanic neighborhoods with resources devoted to white parks.

Washington's appointees took control of the board, stripped Kelly of his power and appointed their own chief executive, a black city official. Alarmed, the mayor's opponents tried an end-run, pressuring sympathetic state legislators at the close of a special session in Springfield to rewrite the park district book.

Washington mounted a counteroffensive, warning legislators that wrath among black voters could hurt the Democrats in their hopes of retaining control of the legislautre in the Nov. 4 elections. The anti-Washington effort fizzled.

Then, in a Keystone Kops rendition of Dynasty-like skulduggery, Kelly locked the new arrivals out of his headquarters, ordered his staff into total silence and seemingly took a vacation.

This week, newly minted Park District President Walter Netsch discovered that someone had mysteriously removed all files and documents from the absent Kelly's locked office. When Netsch had the office's doors opened, Kelly interrupted his vacation long enough to complain to a local television station of "Gestapo tactics."

Undeterred, Netsch announced he will administer civil service exams to parks employes. "We're interested in competence now, not patronage," he said.

Meanwhile, stymied in their efforts to stem Washington's assault on this and other key bastions, the mayor's opponents have launched a petition drive for a referendum on the fall ballot to change the city's spring 1987 mayoral election to a nonpartisan contest.

The nonpartisan system would provide for a runoff between the two top vote-getters. In 1983, Washington won the Democratic mayoral primary when two white opponents, incumbent Jane M. Byrne and Cook County prosecutor Richard M. Daley, son of the late mayor, split the white vote. A free-for-all nonpartisan primary would sharply decrease the likelihood of a similar occurrence.

Anti-Washington leaders disclaim responsibility for the petition drive, which reportedly is near the goal of 143,000 signatures needed by Aug. 18 to put the issue on the ballot.

Although Washington is known to be eager to win another term, he has not announced his plans. Only Byrne is actively campaigning. But the mayor has reacted sharply to the petition drive, denouncing it and blaming his council opponents. "They want to throw a brick and hide their hand," he said at a news briefing today.