Faced with declining contributions from members and private foundations, leaders of the 70-year-old NAACP have decided to rely more heavily on corporate aid for financial support.

That decision was underscored today by an announcement that the NAACP's 1979 fund-raising campaign will be chaired by William M. Ellinghaus, vice chairman and president-elect of the American Telephone & Telegraph Co.

Also, officials of the Mobil Oil Co. said today that, at the NAACP's request, they have sponsored a series of advertisements in the nation's major newspapers urging businessmen and others to contribute to the NAACP, the nation's oldest and largest civil rights organization.

The first Mobil advertisement appeared last week in The New York Times. It is scheduled to run this week in The Washington Post and The Washington Star, The Wall Street Journal, Los Angeles Times, Chicago Tribune, Boston Globe and Christian Science Monitor.

NAACP executive director Benjamin L. Hooks, speaking to reporters after the initial sessions of the organization's annual board meeting here, said he is aware that the Ellinghaus appointment and the Mobil ad campaign will draw charges that the NAACP is "in the pocket of the corporations." But he said those charges would be untrue.

"We're going to the businesses saying: 'Look, you're not giving us anything. You're making an investment in the future of this country.'

"The businesses know that by supporting us they are supporting an organization that is working for change within the system rather than one that is tearing the system down," Hooks said. He added, "some of the more enlightened leaders in race relations are in the business community, especially in the automobile industry... but they understand that we are not obligated to them and that we may have to put a picket line in front of their place tomorrow morning."

Evidence that the NAACP is depending more on businesses for support is shown in a summary of its fiscal 1978 fund-raising efforts, made available to reporters today.

Individual contributions yielded $800,000 in the last fiscal year, compared with an estimated $850,000 contributed by corporations. Some $780,000 came from private foundations, and $247,000 came from "other" sources.

In the previous fiscal year, 1977, individual contributions yielded $978,000; foundations contributed $824,000, corporations $595,000 and others $378,000.

Hooks said today that he would like to maintain at least a 50-50 giving ratio between black and white contributors. Currently about 60 percent of all contributions come from blacks and 40 percent come from whites, he said.

But the gifts were not enough to keep the NAACP from sliding $680,000 into the hole in the last two years, Hooks said. The organization was so financially strapped that it had to mount an emergency appeal for funds in the last three months.

Hooks said the emergency appeal brought in $80,000 from corporations, $100,000 from black churches and another $100,000 from the black community at large. He said he hopes that the Mobil ad will help to raise additional revenue.

"We think the ad was a legitimate response to a very real and very important problem," Hooks said."We had been talking to a number of companies about our emergency appeal, and this is what Mobil decided they could do to help us."

Some NAACP officials said privately today that the Mobil ad used softer language than they would have liked in urging potential contributors to help the NAACP out of ts financial difficulties. Others complained that the ad did not include an address where contributors should send their money. But most were grateful.

A Mobil spokesman said: "We are longtime friends of and contributors to the NAACP.... This ad came about as a result of a special request, and we were delighted to help."