Delegates loyal to President Carter displayed their strength in the Democratic Party platform committee late last night, rejecting 83 to 49 the basic economic proposals put forth by Carter's opponent Sen. Edward M. Kennedy (D-Mass.).

After committee Chairman Coleman Young, Detroit's mayor and a Carter loyalist, established rules to speed debate, the committee heard summaries on a broad range of fundamental economic issues from just four speakers.

Lt. Gov. Thomas P. O'Neill III of Massachusetts made a brief but emotional plea for an immediate $12 billion antirecession jobs program, immediate wage and price controls and the other elements of Kennedy's economic program.

"We're talking about a restatement of values for the Democratic Party," O'Neill said. "How much higher must unemployment go?" he asked.

But the 158 members of the platform committee seemed in no mood for political passion. A buzz of conversation filled the meeting room at the Mayflower Hotel as O'Neill spoke, and the exhausted delegates then voted along predictable Carter-Kennedy lines on O'Neill's motion.

On a series of half a dozen votes on specific elements of the Kennedy economic program, debated after the comprehensive Kennedy plank was rejected, the Carter forces maintained a decisive majority. The closest vote came on a proposal to back only Kennedy's immediate, $12 billion antirecession jobs program, which was defeated 84 1/2 to 57.

The committee then adopted the Carter version of the economic planks by a vote of 92 1/2 to 51.

But there was no sign of the significant erosion in Carter's support that Kennedy backers are hoping for eventually to unravel the president's present lead in delegate votes for the Democratic National Convention in August.

Earlier yesterday, delegates to the platform committee finally got a chance to vent some independent feelings, successfully challenging the dominant Carter forces on nuclear energy and abortion in drafting subcommittees that can still be overruled by the full committee.

Independent-minded delegates pledged to Carter joined those supporting Kennedy to vote for an antinuclear power plank in the energy task force. And in the human needs task force, a coalition of women supporting both candidates won a large majority for an abortion plank that is substantially more "pro-choice" than the Carter campaign would like.

Neither vote is conclusive, and both may be overturned by the full committee before it completes work Tuesday. Nevertheless, these votes symbolized defeats for the Carter campaign organization, and thus heartened Kennedy delegates who are still hoping for some miracle that could undo the verdict of this year's state primaries and caucuses.

The votes on two such highly emotional issues also symbolized the divisions that beset the Democrats this year. So did a tie vote in the foreign policy task force on a proposed plank that opposed construction of the MX mobile missile system. The tie was broken in favor of the MX by the task force chairman, a representative of Democrats Aboard whose ballot has the value of only one-fourth of a vote.

The antinuclear plank approved yesterday demonstrated how a coalition could be formed to override the presidential preference on the platform. The draft plank on nuclear energy, endorsed by the White House, had been essentially pronuclear, calling nuclear power's role crucial in the nation's energy system.

A Carter delegate, Carrie Wasley of Minnesota, proposed the alternative -- calling for an "orderly phase-out" of all nuclear plants and a moratorium on licensing new ones. With the support of Kennedy delegates and active lobbying by the Campaign for Safe Energy, it became aparent that the Wasley alternative had wide support.

Yesterday, before the task force on energy convened, the Carter delegates on the panel caucused with Stuart E. Eizenstat, President Carter's chief domestic adviser.

Eizenstat pressed for a newly drafted alternative to the Wasley proposal that committed the party "to the preferred use of conservation, coal and renewable energy sources for the future," but omitted the moratorium on nuclear power plants and the phase-out of existing ones.

Wasley listened to Eizenstat's presentation and disputed his proposal. "I got angry," she said in an interview, "when he said he didn't know what an 'orderly phase-out' meant." She stuck by her proposal, won over six other Carter delegates and prevailed on four others to abstain. When the 17-to-11 vote in favor of Wasley's plank was announced, a cheer went up in the Mayflower Hotel room where the task force was meeting.

The abortion plank was changed by a strong coalition of women delegates, who gained the support of a lot of men. Instead of the draft platform language, which said the Democratic Party regarded a constitutional amendment banning abortion as "not appropriate," the new language opposes the amendment, and says the party recognizes the 1973 Supreme Court decision on abortion as "the law of the land."

The MX debate also uncovered support among Carter delegates for an antiadministration position. The secretary of the foreign policy task force, picked by the Carter camp, abstained on the vote, apparently because she is from Utah where the missile system is supposed to be deployed and where it has become increasingly unpopular.

The anti-MX plank would have been approved in the task force had it not been for the proxy vote of Rep. Don Bonker (D-Wash.), a Carter delegate who, ironically, has opposed the missile system in the past. Bonker's proxy was held by Richard A. Pinaire of Kansas, who apparently was unaware that Bonker was against the MX. Pinaire said he had been given the proxy without any conditions.