Democratic gubernatorial nominee Charles S. Robb is spending this Labor Day weekend wooing minorities and labor while opposing programs that are dear to many blacks and union members.

Robb, whose campaign strategists have said he will need both union members and blacks to win in November, got away with that seeming contradiction this afternoon at an evangelical church convention where a bishop told his followers, "politicians aren't saints." Robb also was helped by the absence of his opponent, Republican J. Marshall Coleman, who was invited but didn't show up.

Elder Ronald Watson of the Church of God in Christ introduced Robb to the 2,500 delegates here as "our next governor and a future president of the United States."

Bishop Samuel L. Green Jr., who heads a church with 64 congregations and 25,000 members in Virginia, said that he would like to see Robb come out in favor of extension of the Voting Rights Act, but told the congregation "we understand why" he opposes it.

The gravel-voiced bishop, whose followers screamed in frenzy during his shouting, hand-clapping, singing sermon, said, "People all over the state should vote for Chuck Robb. Legislators enact laws that affect our lives. We must get involved politically." Green, whose influence is helped by a weekly program televised by the Christian Broadcasting Network, said Robb is "the best candidate for all of the people, not just a segment."

As to Robb's views on the Voting Rights Act and other issues on which he is at odds with many blacks, the bishop said "We'll deal with that later. I'll explain that to you later on."

In his brief talk before the church convention, Robb said he and his running mates "need your help and your prayers."

Robb pointed out that he had asked Green to deliver the invocation at his inauguration as lieutenant govenor four years ago and at this spring's Jackson Day dinner in Richmond.

Robb told the black churchgoers of an exchange he had yesterday in Alexandria with several young blacks. One of them, Wade Mitchell, 26, an employe of the Alexandria Police Department, told Robb he is "living from paycheck to paycheck" and complained that President Reagan "is doing a lot of cutting" of social programs.

Another said, "I don't want welfare, I want a job," and asked Robb what he would do for the economy.

Robb, momentarily flustered, said, "I've got a 50-page document on economic development , but it's more than you want to know." Then he added that he favors a program begun in Delaware for increasing minority employment, but said "We must create the demand within the private sector."

Robb told the convention today that his questioners "were getting pretty critical" and pledged his concern for minority problems.

Del. Bobby Scott (D-Newport News), one of four blacks in the 100-member House of Delegates, told the convention that "if you want Martin Luther King's birthday to be a state holiday, you must vote for Chuck Robb." Scott noted that Coleman had supported Gov. John N. Dalton's veto of such legislation earlier this year.

Robb, lieutenant governor candidate Richard Davis and attorney general nominee Gerald Baliles, left the stage before the bishop's main address, but they were there long enough to hear an exciting chorus whose rhythmic renditions had even the placid Robb on his feet clapping hands.

At his appearance in Old Town Alexandria yesterday, Robb told a rally that he was encouraged by the solid support of black elected officials and growing backing from rank-and-file union members.

But while Robb was boasting of attracting support from those traditional Democratic constituencies there was a sign of liberal dissatisfaction within the crowd.

Ron Fitzsimmons was circulating a resolution that he said he will introduce Thursday night at a meeting of the Alexandria Democratic Committee that says Robb's conservative views are making it "exceedingly difficult, if not impossible, to publicly and actively support" his candidacy.

Fitzsimmons, a liberal who was an aide to former U.S. Rep. Herbert H. Harris, said he already has received the support of about l5 members of the 75-member committee. Fitzsimmons said "the last straw" was a letter Robb sent to President Reagan endorsing the Republican tax-cut plan.

Robb dismissed the resolution, asking "Who is he?" Robb said that even though he failed to get the endorsement of the state AFL-CIO at its meeting last month, many union members "respect my right to disagree" on such issues as the right-to-work law, which he favors, and collective bargaining for public employes, which he opposes.

"I'm trying to tell everybody the same thing," Robb said. "I'm not making a deal with any single element."