Urban League president Vernon Jordan last night challenged the Reagan administration to explain what will happen to poor people during the "interregnum" after social programs are cut next fall and before the promised upswing in the economy occurs.

"Even the wildest optimist knows it will take years for the president's program to produce the prosperity he promises," Jordan said. "What do we do until then? How do poor people survive without the basic programs they need until then?"

It was Jordan's opening challenge in a debate he has attempted to engineer this week with key administration officials over the deep reductions in federal spending for social programs. Vice President Bush, Office of Management and Budget Director David A. Stockman, United Nations Ambassador Jeane Kirkpatrick and several other members of the cabinet are to speak for the administration to the anticipated 15,000 who began pouring into Washington this weekend for the Urban League's annual conference at the Sheraton-Washington Hotel. Former vice president Walter Fauntroy are among Democrats on the schedule who are expected to argue against the administration.

In a round of press briefings and in his opening speech last night, Jordan repeated his objections to the economic proposals, calling them a "jellybean budget" that enriches the wealthy at the expense of blacks and poor people.

But, there was a clear contrast in his style and that of NAACP board chairman Margaret Bush Wilson who introduced President Reagan at her group's annual meeting in Denver earlier this month by announcling, smilingly: "The NAACP does not necessarily subscribe to the views that are about to be expressed." White House aides, regarding that as bad from and a gratuitous slap, were stunned and angered. Clearly Jordan is seeking to avoid fostering that kind of antagonism here this week.

While he did not flinch at strongly criticizing the administration, describing it in his speech as both lacking in compassion and "wedded to an ideology of radical conservatism" that has been outmoded since the Great Depression, Jordan was careful to avoid personally attacking Reagan.

"We must make a clear distinction between our political and ideological differences with President Reagan, on the one hand, and our high personal regard for him on the other," Jordan said. "The president is a good man, a courageous man and, on a personal level, a compassionate man."

Acting to set the terms of the administration's response in its speeches to the league this week, Jordan attempted to deflate many conservative arguments used to justify the deep spending reductions. He disagreed that most social programs had failed or had bred dependency. Most blacks are anxious to enter the economic mainstream, he said, and "we know that will not happen without a federal government that pushes the private sector into affirmative action programs."

He acknowledged that blacks who are to be disproportionately affected by the proposed cuts are isolated, without many old allies and forced in the present climate to again fight battles for laws like the Voting Rights Act which "we fought 16 years ago."

"We are thrust back to square one."

He criticized Democrats in Congress for "passively" going along with the new conservative mood and at one point suggested to his audience that their problems were created both by "weak-kneed liberals and hard-hearted conservatives."

He called for raising "high a fresh banner of protest" but provided little else in the way of a strategy for his listeners to follow at this late stage in a budget fight that is all but over.