Under a pressure from the White House, Sears, Roebuck & Co. announced yesterday that it will withhold payment of a $25,000 pledge to the U.S. Olympic Committee unless it formally agrees to boycott the Summer Olympics in Moscow.

Coming a week before the USOC's House of Delegates is to meet on the boycott issue, the Sears decision appeared to be a blunt warning to the committee of the financial consequences of a decision to defy President Carter's demand that the United States not attend the Moscow Games.

F. Don Miller, executive director of the USOC, called the Sears action and the White House role in obtaining it "sheer blackmail."

Official of Sears, a major corporate contributor to the USOC, said the decision on withholding the $25,000 resulted from a request Wednesday from senior presidential assistant Anne Wexler to Sears board chairman Edward R. Telling.

Wexler acknowledged that she and Telling had discussed the Olympic boycott and Sears' role as a contributor to the USOC, but she denied requesting that the payment be withheld.

"I didn't suggest it or ask him, and he didn't offer, she said.

However, shortly after speaking with Wexler, Telling decided that Sears would hold up payment of the last $25,000 installment in its $75,000 pledge to the USOC pending the outcome of the boycott dispute.

Confirming this, Telling yesterday issued a statement from the company's Chicago headquarters.

"We feel that in matters of international affairs such as this, the responsible position is to support the president," he said. "We have notified the Olympic committee of our decision not to remit the third installment of our three-year pledge unless the committee publicly announces it will not participate in the Moscow Olympics."

Under international Olympic rules, the USOC has until May 24 to accept an invitation to attend the Moscow Games. Seeking to stem growing sentiment against a US. boycott, the administration in recent days has stepped up its pressure on the USOC to comply with Carter's demand.

White House counsel Lloyd Cutler who is in charge of the boycott effort, said yesterday there is not orchestrated attempt by the White House to put financial pressure on the USOC.

"We certainly appeal for business support on the Games, but certainly not in terms of cutting off contributions," he said.

But Cutler also said that a number of business executives have "indicated pain" over the reluctance of some Olympic officials to go along with the boycott and he predicted that the USOC would suffer financially if it votes to attend the Games.

"They [corporate executives] may be expressing themselves," Cutler said.

While advocating the boycott in response to the Soviet Union's invasion of Afghanistan, the president has also publicly urged Americans to continue contributing to the USOC, which maintains two training centers for athletes and conducts other activities in non-Olympic years.

"An excellent way for all of us to express our pride in our young athletes is to continue and to increase our financial and our moral support of the U.S. Olympic Committee," Carter said at a Feb. 25 ceremony welcoming members of the U.S. Winter Olympic team of the White House.

Before he learned of the Sears decision, the USOC's Miller said that Cutler and other officials had suggested to the committee that a refusal to support a boycott "would have a negative impact on our fund-raising."

Asked if he considered this a threat, Miller said, "I wouldn't want to call it a threat; the message was there."

In the face of the boycott controversy, the USOC has begun experiencing fund-raising difficulty.

According to Ray Mueller, its director of fund-raising, before the Afghanistan invasion and the president's call for a boycott, the USOC was exceeding its budget projections, which called for it to raise $43 million between the 1976 Olympics and the 1980 Moscow Games.

But Mueller said that in the first three months of this year, when the USOC expected to raise $4.2 million, $2.6 million in contributions was received.

Most of the falloff occurred because a number of state Olympic committees postponed fund-raising events pending a decision on the boycott, he said.

Mueller called the Sears decision " a disaster," particularly if it spreads to other corporate and foundation supporters of the USOC. He said that up to $3 million in corporate and foundation pledges to the committee are due to be paid this year.

Wexler was vague about the substance of her telephone conversation with Telling. She said he expressed "concern" over reports of growing antiboycott sentiment in the Olympic movement and that they then discussed Sears financial support for the USOC.

The USOC's House of Delegates will meet April 11-13 to discuss the boycott issue, although it may put off a vote until the May 24 deadline for accepting an invitation to Moscow. USOC officials previously have said they believe the committee will support the boycott.