The strain of the campaign began to show today on the Democratic mayoral candidate, Rep. Harold Washington (D-Ill.).

Washington flung a scatological label at a student who asked about his tax troubles, missed a radio talk-show and a businessmen's forum where he was expected, and lashed out repeatedly at his opponent, Republican Bernard E. Epton, for allegedly keeping the race issue alive in the campaign.

The uncharacteristic outburst from Washington, 60, came amid fresh reports of infighting in his campaign organization and reflected the pressures Washington and his aides are feeling as they nurse what they say they believe is a narrow lead through the final days leading up to Tuesday's election.

On the basis of nightly telephone polls, Washington's pollster, Patrick Caddell, said today that there has been little movement in the past week but that he expected the large clumps of undecided white voters, clustered in independent-minded lake-front wards, to "start breaking" in the next two days.

A new poll broadcast tonight by the local CBS-TV affilitate, WBBM, showed Washington leading Epton by 51 to 38 percent, with 11 percent undecided. The poll, taken Tuesday night, also showed Washington's support in the white community falling from 29 to 21 percent in two weeks, and undecideds turning to Epton by more than 2 to 1.

Washington, who had campaigned until late Wednesday night, started today on a bad note, missing a scheduled 90-minute date on the city's top-rated morning radio call-in show. Washington aides said the appearance had never been confirmed, but host Wally Phillips and his producer told his audience Washington had promised to be there.

Later, about 100 members of the business leaders' prestigious City Club were left waiting for more than an hour past Washington's scheduled arrival time before an aide came to say Washington could not make it. Thomas Roeser, president of the organization, said the members were "very hurt."

During the last few days, as both campaigns focused on the undecided white voters who apparently will determine whether Washington will become Chicago's first black mayor, a series of seemingly timed leaks have hit the Chicago papers about debts that Washington left unpaid in Chicago and during his years in the Illinois Legislature.

After a midday appearance at Northeastern Illinois University, Washington lashed out at Epton for "bringing up picayune issues from the past" and telling "base lies."

As Washington was about to enter his car, a young person in the crowd hollered, "Hey, Mr. Washington, how come you didn't pay your taxes?"

"Go to hell," Washington said.

When the youth repeated the question, Washington shouted, "I didn't go to jail for not paying taxes obscenity ."

He later said he regretted the outburst, and explained that he lost his cool because, a moment earlier, an aide had handed him a piece of hate literature that depicted him as a child molester. "I've had enough," he told a group of supporters on the North Side. "To be assaulted on my fundamentals, my macho, will not be tolerated."

Washington was convicted of failing to file income tax returns for five years and served 36 days in jail in 1972.

Senior Washington aides talk of frustration over a seeming inability to mediate disputes involving Edwin C. (Bill) Berry, Washington's transition-team chairman and personal adviser; William F. Ware, his top congressional aide; Clarence L. McClain, his senior Chicago office lieutenant, and many of the people who put together the "people's campaign" that powered him to an upset victory in the February primary.

But Washington is heavily scheduled in the closing days in the white lake-front wards, and tonight his campaign regained the right to use a commercial aimed at that vote.

By paying CBS News $450 for rights to its tape, Washington's media adviser, William Zimmerman, cleared away the legal barrier to a commercial that is as much a plea for tolerance as for votes.

The spot contrasts the angry faces of pro-Epton picketers, who taunted Washington and Walter F. Mondale outside a white neighborhood Roman Catholic church two weeks ago, with the faces of young children of both races reciting the Pledge of Allegiance.