The Carter administration, backed by bipartisan congressional support, came out swinging yesterday in its campaign to force a final and favorable decision this weekend to boycott the Moscow Olympics.

At a State Department briefing yesterday, administration officials were highly critical of delays by the U.S. Olympic Committee, which has been deferring a vote on rejecting an invitation to compete at the Games this July.

"We made a fatal assumption back in February," said State Department spokesman Hodding Carter, "that athletic organizations -- no less than the farmers, no less than businessmen, no less that people interested in cultural or other exchanges -- were going to believe that business as usual could not be the order of the day. . . .

"There has been a massive, if not well publicized, campaign to face down not only the president of the United States, but both houses of Congress which voted overwhelmingly in favor of a boycott and public opinion as measured by all the polls," including one of the USOC's.

With a possible USOC vote on the boycott issue only five days away, administration officials acknowledged that they had miscalculated the anti-boycott sentiment in the committee and were pressing to reverse it.

The USOC House of Delegates will meet this weekend in Colorado Springs, where it will be asked to decline an invitation to participate in the Moscow Games because of the Soviet invasion of Afghanistan.

They have been strong indications, however, that the delegates may refer THE QUESTION TO THE USOC's administrative committee for action in mid-May, shortly before the May 24 deadline for sending U.S. Olympic entries.

Further complicating the situation is the mounting opposition to a boycott by the national governing bodies of the Olympic sports, which control 71 percent of the USOC House of Delegates.

The adminstration has been lobbying representatives of those governing bodies and other USOC voters during the past week at State Department briefings, the last of which will be hel today. President Carter also began sending telegrams to USOC voters yesterday, seeking support.

The telegram said, in part: "The continued aggression and brutality in Afghanistan has shocked people around the world and threatens peace and stability in the world. In these circumstances, to send a team to Moscow would be against the national interest and against the interest of national security."

Spokesman Carter said a further delay by USOC would be a disservice to athletes and to plans for substitute competition.

Athletes, he said, would be "held out there in a form of truly unfair suspense -- i.e., that there's going to be some miracle to save them. Whether a decision comes now or at the end of May, the fact is an American team is not going to be going to Moscow."

"There is a tendency in some quarters of the USOC, encouraged by a number of International Olympic Committee figures, to believe a delay can be used to mobilize massive pressure for a reversal of our decision," Carter said, adding, "The longer we wait to make the decision, the harder it will be for others to decide not to tgo to Moscow. Those who don't want a boycott are counseling the USOC to delay."

Asked if he expected an antiboycott vote, the State Department spokesman replied, "No, I honestly don't believe athletic organizations will walk up and say, 'Ladies and gentlemen, because athletics mean so much to us, we're going to forget about Afghanistan, repudiate the president and Congress and give a nice pat on the back to the people of Moscow.'"

Deputy Secretary of State Warren Christopher said a boycott "clearly is one of the most important tools we have to put the Soviets' feet to the fire."

"There are times when individuals and a nation must stand firm and this is one of them . . .," Christopher said. "The right thing to do is for the Olympic Committee to decide next weekend to honor the president's position."

White House counsel Lloyd Cutler and deputy counsel Joseph Onek urged the USOC to decide on a boycott this weekend and decline its invitation immediately. Such action would prompt other countries to follow suit, they said.

Failing that, Onek suggested, the USOC could "indicate" that its members have decided to boycott, but delay sending a formal rejection. "That would be less acceptable," Onek said, but would achieve the same result.

Should the antiboycott faction prevail, Onek said, the administration has "a variety of legal resources at its disposal!" to block the fielding of any U.S. teams, although the president would not prohibit individuals from competing.

He refused to discuss specifies, explaining that the administration did not expect to have to use them "because we expect the USOC will live up to its responsibilities."

In Congress yesterday, House and Senate leaders sent strongly worded bipartisan letters to the USOC urging it to boycott as an example to "other freedom-loving nations."