Rep. Harold Washington, Chicago's first black Democratic nominee for mayor, has built a reputation as a thoughtful legislator, a forceful civil rights advocate and a dynamic, witty orator during two years in Congress and 16 in the Illinois Senate.

He spent most of his career working within the political machine built by the late Mayor Richard J. Daley before breaking openly in 1977 in frustration over its refusal to share power with independent black politicians.

In a city with a history of segregation, Washington's mayoral campaign was rife with race-oriented oratory.

"It's our turn," he would tell black audiences. "We're not going to apologize for it and we're not going to waste a lot of time explaining it. It's our turn--that's all."

He has never been been reluctant to use his oratory as a sword. In 1981, during hearings on the Reagan budget, he told David A. Stockman, director of the Office of Management and Budget, that administration policies were "exacerbating, perpetuating, incubating, fomenting revolution. These cities are going to blow."

Behind the sometimes hot rhetoric, associates say, stands a proud, dignifed, almost diffident man and a highly pragmatic legislator.

In his short tenure in Congress, Washington won high marks from the civil rights community for his leadership in the successful effort to extend and strengthen the Voting Rights Act.

"He came to every hearing, always did his homework, and I think knew the substance of the issues better than anyone in Congress," said Ralph Neas, executive director of the Leadership Conference on Civil Rights.

In Springfield, Washington led the Illinois fight for legislation that established the state Human Rights Commission and which made Dr. Martin Luther King Jr.'s birthday a state holiday. One poll of legislators and capitol correspondents named him one of the state's 10 most effective legislators.

Washington, 60, was born to politics. His father was a Democratic precinct captain on Chicago's Southside, a black section that has middle-class as well as poor neighborhoods, and young Washington inherited the precinct on his father's death in 1954.

By that time he had made a name for himself in other areas. He won the city championship in the 120-meter high hurdles and compiled a 60-10 record as an amateur boxer. At Roosevelt University he was elected president of a senior class that was only 5 percent black.

Washington was married young, divorced more than 25 years ago, and has no children. He is now engaged to a Chicago teacher, and has recently moved from the Southside to an apartment in the more affluent Hyde Park area.

He has had two bouts with the law. In the 1960s he was temporarily suspended from the bar by the Illinois Supreme Court for taking fees from clients and not performing work.

And in 1972 he was convicted of not filing his income tax for four years. His paychecks had taxes withheld during those years, so the amount of underpayment wasn't much--$508.05. Still, Washington spent 40 days in jail and was fined $1,036.

He says he was singled out by Republican prosecutors in retaliation for his leading a walkout in the state legislature when Vice President Spiro T. Agnew spoke there in 1972.

Since serving time in the Cook County Jail, Washington has returned every year to visit inmates. His visit last Christmas Day was one of the more bizzare and, associates say, foolish episodes of the campaign. One television station dug out his old prison photo and began its report: "Harold Washington had something of a homecoming yesterday."