John E. Jacob, president of the National Urban League, opened the group's convention here last night by warning the Republican Party that if it wants to keep the White House in 1988, it "can't continue to operate as if black people don't exist."

Jacob said relations between black Americans and the president have broken down, and the "time is ripe" for the administration and the GOP to begin responding to overtures from black leaders to discuss how to allow blacks to "make a constructive contribution to the functioning of government programs."

Speaking to about 4,000 people, Jacob said in prepared remarks that under the Reagan administration, a "philosophy of selfishness and meanness" has become entrenched nationally. While President Reagan talks about a "colorblind society," more blacks are being forced "deeper in poverty," he said.

In the keynote address for the league's 75th anniversary convention, Jacob said the administration's strategy has been to cut taxes while increasing defense spending. He said those moves have created an "excuse for cutting social programs."

He blamed the Reagan White House for the breakdown in communication between the president and blacks. "I want to point out that it wasn't black leadership that refuses to communicate," he said. "We tried."

He added that communication between Reagan and blacks disintegrated after the president and administration officials accused black leaders of misleading black voters into opposing Reagan to stir discontent and keep their jobs.

"This administration will be around for another 3 1/2 years," Jacob said. "But we will be around for a lot longer.

"So the administration can't continue to operate as if black people don't exist. The Republican Party, looking ahead to 1988, can't allow further alienation of black voters. And anyone who thinks the civil rights coalition is totally powerless, even in this Washington of 1985, ought to ask Assistant Attorney General for Civil Rights William Bradford Brad Reynolds if he agrees."

Reynolds' nomination as associate attorney general, the No. 3 post in the Justice Department, was rejected by the Senate. He turned down an invitation to speak at the Urban League convention.

Jacob called for a "new relationship with this administration -- regular, ongoing channels that allow black leadership access to the people who run the programs that affect our constituents."

He said he is seeking to repair the ties although he said he knows that "We aren't going to change their basic policies. But we can make a constructive contribution to the functioning of government programs."

Jacob called for full employment, a voluntary National Service Program to train young people, and a national welfare program that "allows poor families to maintain a minimum living standard so poor children can eat and have roofs over their heads."

"To those who complain that these are old ideas," he said, "we answer that they are good ideas, ideas that have not been tried, ideas that offer hope.

"What's being sold today as 'new ideas' is really just a new bottle to hold the old poison -- the poison of neglect, of meanness, of throwing human beings into the ditch of permanent poverty."

He also called for a select committee in Congress to examine defense spending and "get the Pentagon off the backs of the American people."

Jacob reaffirmed his group's support for affirmative action and said the administration's opposition to race-conscious remedies is part of a "reactionary world view that says . . . if discrimination exists, it's black people's fault."

At a news conference yester- day, Jacob announced plans for a large demonstration Tuesday at the South African Embassy. He said some members of the Urban League plan to be arrested.