District of Columbia Mayor Marion Barry went to church on Chicago's South Side yesterday, urging blacks to forgive Democrat Harold Washington for his past legal problems and elect him mayor on Tuesday.

"You find someone who has not sinned once or twice and you've found someone who is not living," Barry told about 400 members of the Third Baptist Church of Chicago, a middle-class black congregation.

Barry arrived here early yesterday for a three-day stay to help Washington, a congressman and former Illinois state legislator, in his bitter, hard-fought race with Republican Bernard Epton. Barry also hopes to enhance his own national political profile.

The mayor repeatedly sought to minimize the importance of Washington's 1972 conviction for failing to file federal tax returns for four years and a steadily growing list of other legal problems that Epton has sought to exploit.

"Let us not be myopic," Barry added, as many of the well-dressed church members encouraged him on with "amens."

"Let us [blacks] not stab each other in the backs."

Later, at the Shiloh Baptist Church, Barry told another hand-clapping congregation not to be confused "by all the confusion about what Washington did 15 years ago."

The mayor also sought to play down the significance of racial fear in the campaign--a difficult task given the bitter racial rhetoric that has issued from both camps.

Barry said that Washington's opponents really fear that he will put an end to Democratic machine politics and patronage as Chicago has known them for decades.

"Whenever you start reform, the people who took it all from you for years get upset," Barry said.

Barry, who has ties to Chicago through his family and years of work in the Civil Rights Movement nationwide, was greeted like a favorite son by the Rev. Elmer L. Fowler of Third Baptist and the Rev. C. Jameson Brooks of Shiloh, a cousin of the mayor's.

"He's clean cut and people who know him around the country respect him," said Fowler.

But it was clear throughout the day that Barry has his work cut out for him in trying to enhance his national profile. When asked prior to Barry's arrival at Third Baptist what she thought of Marion Barry, one parishoner replied, "Oh, is she speaking here?"

Loretta Coleman, a city employee and member of the Third Baptist congregation, said that while she doesn't know much about Barry she felt it was important that he had come to Chicago on Washington's behalf.

"It says there is unity in the whole nation and that others are concerned," said Coleman.

In sending Barry into south and southwest Chicago yesterday, Washington was merely showing the flag to his supporters while he concentrated on far more crucial areas, including the integrated north lake-front area. Many observers believe that area will be decisive in the outcome of a close election.

Washington said recently that Barry would be helpful to him in the final days of the campaign because he appeals to reform Democrats who came through the Civil Rights Movement with Barry and who are impressed that one of their own was elected mayor of a major city.

"The value of people coming in isn't measurable," Washington said, "but it's there."

Others who were here over the weekend to campaign for Washington included Reps.John Conyers Jr. (D-Mich.), Harold Ford (D-Tenn.) and Ed Roybal (D-Calif.), and Mayor Milton Tutwiler of Winstonville, Miss.

Barry was accompanied on his flight here by attorney Woodrow Boggs and two D.C. police bodyguards. Deputy Mayor Ivanhoe Donaldson, Barry's top political adviser, flew here Friday to help Washington's fractious campaign organization. At least part of the trip expenses are being paid from D.C. city funds.

Barry also spoke yesterday to a group of Chicago Muslims and took part in rallies sponsored by Latino artists and a women's group. He appeared with Washington last night at a spirited rally at DuSable High School. Barry and Washington also addressed about 200 white Washington supporters at St. Xavier College in far southwest Chicago.