Rep. Harold Washington (D-Ill.) is riding a grass-roots crusade as much as he is leading a political campaign.

The marriage was a perfect match for the Feb. 22 primary, but in the ensuing seven weeks it has played havoc with his quest to become Chicago's first black mayor. The election is Tuesday.

At a time when many expected him to reach out to the whites who dominate the Democratic Party here, Washington opened the short general election campaign by sounding the same it's-our-turn and anti-machine themes that fueled his surprise primary victory over Mayor Jane M. Byrne and Cook County State's Attorney Richard M. Daley.

Washington demonstrated a streak of hubris in not immediately forgiving former vice president Walter F. Mondale for not supporting him in the primary. He said he wouldn't "grovel" for support from the Democratic political leaders who had opposed him, and he got into a sharp public exchange with the popular police superintendent, Richard Brzeczek, who has since submitted his resignation.

More recently, however, the tone of the Washington campaign has become inclusive. His standard stump speech is a hands-across-the-neighborhood appeal in which he talks eloquently of the common problems that bind all Chicagoans.

But it is clear from the polls that white Chicago has digested the first message more fully than the second. Tuesday's election will turn on whether Washington can get the 20 percent of the white vote he needs to win.

The initial hard line, Washington advisers said, was the product partly of calculation and partly miscalculation.

"Harold was very determined not to repeat the Jane Byrne syndrome of attacking the machine and then jumping into bed with it," said one campaign aide who asked not to be identified. "He wants to be its leader, not its captive. He wants to deal from a position of strength."

If he is elected, Washington will have to work in a strong city council form of government in which a majority of the 50 aldermen will owe their allegiance to the Democratic machine.

"You've got a core of 16 black aldermen and you've got some whites who will be with me," Washington said in an interview just after the primary. "Now you're getting close to a majority. You don't need a majority-- you need just to get close."

But while Washington was positioning himself for battles over budgets and patronage, he was also overlooking the possibility that his lightly regarded Republican opponent, former state representative Bernard E. Epton, could develop into a serious challenger. That was the miscalculation.

"There was a tremendous amount of anger, even in victory," said another Washington campaign aide. "We really weren't ready to win."

The Democratic nominee may indulge at times in some of that anger, but he knows it cannot run a campaign or a city government.

"Harold strikes me as a pretty pragmatic politician," said Milton Rakove, a local political science professor and expert on the Chicago machine. "In the days when he was with the machine, that's where the ducks were lined up in the black community. When things started to break away he followed, he didn't lead."

Caution has been Washington's hallmark throughout the three decades he has been in politics. He was a reluctant candidate for mayor, telling supporters that he would not get into the race until the arithmetic of voter registration and turnout changed.

Another source of Washington's reluctance was that in 1980 he achieved his life-long ambition of getting elected to Congress from Chicago's South Side.

"He viewed that as the mountain, and wondered why he should get into a dogfight," recalled Dempsey Travis, a friend from their days at Roosevelt University. Washington, he said, has envisioned himself using his seat in Congress and his oratorical skills to emerge as a national spokesman for blacks.

Washington had not spent enough time in Congress to leave much of a mark, and his record from Springfield, where he served 16 years in the legislature, is mixed.

"He seemed to find himself once he broke from the machine. He seemed to get more respect for himself," said state Rep. Daniel Pierce of Highland Park.

Washington grew up with politics. Both his father and grandfather were Methodist ministers. His father was also a lawyer and precinct captain, as, eventually, was the son.

"I wasn't good enough to be a minister," the candidate tells audiences, "so I became a politician instead."

With a deep calming voice, an infectious personality and a zest for the game, he is a natural on the stump.

The campaign also has brought him back to the church, a traditional pulpit for politicians in the black community, where he describes blacks as being "moral supermen" whose "manifest destiny" is to heal racial tensions.

"We are the prophets, the soothsayers, the healers, the unifiers," he said. "We have the tolerance, the courage, the understanding and the humanitarianism to heal this city and save it no matter what the strident voices say."

But if Washington uses his race to make an appeal for unity he also has used it to deflect the attack on his failure to pay bills and file tax returns.

"It bothers us that we should have to explain ourselves over and above the ordinary explanations demanded of people in public service," Washington told the same congregation at the Carter CME church on the South Side. "Your candidate isn't perfect, but . . . his sins, minor though they be, are behind him."

He accuses Epton, the Republican candidate, of making racial appeals, and attributes a mock quotation to him: "Anything goes. Old Harold can take it, he's black, so we can get away with it."

Washington's record of failing to meet financial obligations suggests a pattern of sloppiness, perhaps even a willful flouting of authority, but not greed. The consensus of friend and foe is that Washington has never been on the take.

He was convicted in 1972 for not filing federal tax returns for four years. He contends that the total amount owed was small--$508--and that the prosecution and subsequent 40-day jail sentence were politically motivated.

But the assistant U.S. attorney who prosecuted the case said in court that Washington hadn't filed his tax returns for 19 years and that only the statute of limitations kept the government from broadening the charge. Washington denies that, but has not produced any documents.

The events leading to his suspension from the bar from 1970 to 1976 have a similar cast. Five clients had accused Washington of taking small retainers of $150 or less to handle divorce or traffic tickets and then failing to perform. The disciplinary board of the state bar twice subpoenaed him to hear his side of the story, and both times he failed to appear. That, as much as the charge, led to the suspension.

But his tax history at least has provided one bad news/good news gag in this nasty campaign: "When Harold Washington is elected mayor, taxes are going to go up--but no one is going to have to pay."