The National Urban League, which celebrated its 75th anniversary at its annual convention here last week, is facing an era of financial difficulties that threaten its ability to assist black Americans.

Over the last four years, the league has seen its $30 million in government and foundation grants and contracts cut in half. Its general fund for basic staff and operations has remained at about $7 million for three years.

Squeezed by inflation, the league reduced its staff by 15 percent last year. It also opted not to make its 1985 payment into its employes' pension fund.

This year the league, like the NAACP, finds itself beset by financial troubles amid debate in its shrinking ranks over what it should be doing. Collectively, the groups' troubles may signify a sharp setback for the civil rights movement that flowered in the 1960s.

The league, for instance, finds its national profile in decline. President John E. Jacob, a thoughtful but low-key person, has not been a media star of the magnitude of former National Urban League president Vernon Jordan. And the combative relationship between blacks and the Reagan administration has denied Jacob a place on the national stage as an adviser to the president and Cabinet on key issues.

Consequently, the league has found it necessary to attend to its image. It has formed a "board of external affairs," which will "develop a PR package -- recast our image for today," according to Frank Lomax, its executive vice president.

Other problems have stemmed from the Reagan administration's tough talk on civil rights. Some of the league's white corporate contributors have asked it to soften its criticism of administration policy.

"The way we relate to white corporations is usually around planning how to serve our constituency," said Jacob. "We've never had differences with them on our [social service] programs. But increasingly some get disturbed over our advocacy. The position we take . . . in our advocacy is a requirement because of the constituency we represent: poor, black Americans. And we have no intention of abandoning it."

Despite the problems, there were signs of vitality at the league convention, which was attended by more than 10,000. The well-heeled crowd included some young people, a group not in evidence at the NAACP's recent gathering.

A few participants in their 20s and 30s said the league provided them with business contacts, both with big companies and among black professionals, and a chance to be involved in civil rights.

"I've been looking at jobs, networking with some of the other people here," said Stacy Green, 22, of the District of Columbia. "These days nobody is throwing money at black people and offering jobs, so you got to use organizations like this and people you meet to get ahead."

Showing other signs of vitality, the league released a journal of opinion on the state of black leadership and invited a critic of black leadership to speak.

"It is reasonably clear [today]," said Glenn C. Loury, an economics professor at the Kennedy School of Government at Harvard, "that civil rights strategies alone cannot hope to bring about full equality."

Loury argued that more government aid and affirmative action cannot help better prepare young blacks for schools, keep young black women from having children or improve black neighborhoods.

His speech reflected rising pressure within the league to emphasize social problems that can be solved within the black community such as teen-age pregnancy and crime.

"The overall impact on programs of the budget cuts is we will have to be more thoughtful in our advocacy, less vocal," said Carol Gibson, national programs director for the league. "We can't afford the time and effort now. We've had to set our priorities . . . . Our real purpose is to serve the people who walk in the door of our [113] affiliates around the country."

"The National Urban League is not going away," said league Chairman David T. Kearns. "We will have to change and we are changing. But there is a need for the Urban League. It is a bridge between black and white, between nonprofits and corporations for profit, those who don't have resources and those who do . . . . The challenge we're meeting is organizing to help people . . . and that's our priority within the limits of our resources during this tight [financial] period."