President Carter yesterday postponed a news conference that was scheduled for Wednesday until after his beleaguard budget director, Bert Lance, testifies before a Senate committee that is investigating Lance's complex personal finances.

Attempting to fend off questions about Lance from reporters in New Jersey on Saturday, Carter had said he would hold a news conference on Wednesday. But yesterday his press secretary, Jody Powell, said it had been decided to delay the news conference - certain to be dominated by questions about Lance - until after the budget director appears before the Senate Governmental Affairs Committee on Thursday.

No date has been set for the news conference, but Powell said it would be soon after Lance testifies.

The President's last news conference was Aug. 23. He has pledged to hold two news conferences a month, and until the Lance affair had held to a scheduled of one every two weeks.

The postponement, however, did not diminish questions about Lance or speculation that he will resign during or immediately after his appearance before the Senate committee.

At a breakfast meeting with reporters, Greg Schneiders, the White House director of special projects, said there is an atmosphere of "gloominess" among presidential aides because of the Lance controversy and that some aides believe it is almost inevitable that Lance will resign.

But Schneiders said that Carter is not among those who believe a resignation is unavoidable. "He is truly keeping an open mind," Schneiders said.

Schneiders stressed that the decision to stand behind Lance at least until the Senate hearing was the President's and "not a case of bad advice from others." He said he supports that decision although, in somewhat similar circumstances gave up a higher position in the White House.

Schneiders was due to be Carter's appointments secretary. But when questions about some financial transactions in his background were raised, he stepped aside until they were cleared up, and he ended up as the special projects director.

Schneider also said that whatever politicial damage has been done to the President because of the Lance affair is not "irreparable."

At his regular briefing, Powell said he, White House counsel Robert Linshutz and Hamilton jordan. Carter's chief political adviser, all knew before Lance's confirmation hearing in January about more than $500,000 in bank overdrafts by Lance and members of his family.But Powell said that none of the three thought the matter serious enough to bring to the President's attention.

Powell conceded the aides' judgement may have been colored by their friendship with Lance, but said they took full responsibility for the decision.

"A lot of us have spent a lot of time thinking about what could have been done to avoid what has happened," he said.

In other developments yesterday the White House said the President has instructed federal agencies to double their purchases from minority business within two years.

Carter also discussed the administration's human rights policy with Archbishop joseph L. Bernardin, president of the National Conference of Catholic Bishops, and Bishop Thomas kelly, general secretary of the U.S. Catholic Conference. the bishops, according to Powell, said they were willing to testify in behalf of congressional approval of the Panama Cannal treaties.