The Democratic Party's platform committee yesterday completed work on a statement of principles for the 1980 campaign after narrowly voting to endorse the MX missles system.

Senior aides to President Carter who participated in the drafting process welcomed the final result as a "progressive" document that "upholds the traditions of the Democratic Party," and also represents significant compromises with delegates supporting Sen. Edward M. Kennedy (D-Mass.).

"The party can unify" on this platform, said Stuart E. Eizenstat, Carter's chief aide on domestic policy.

Officials of the Kennedy campaign demurred, and promised to carry their central disagreements with Carter policy to the floor of the Democratic National Convention in August.

The most excitement on the final day of deliberations came on the motion of R. P. (Joe) Smith of Oregon, a Carter delegate, to remove any support for the MX missile from the platform and declare instead that the party will "oppose deployment" of the missile system.

This motion provoked two of the most carefully listened-to speeches of the platform deliberations. The first, by Smith, was an emotional plea to the delegates to turn away from another spiral in the strategic arms competition.

"The obvious question is, where will it end?" Smith asked. "But the more important question is, where will we begin to end it?" The experts who favor MX should not be given a free hand, he agrued: "The Maginot Line was built by the experts. The Vietnam war was built by the experts. The MIRV missile was built by the experts," Smith said. "But ultimately the country belongs to the people, and this is us . . ."

David Aaron President Carter's deputy national security adviser, replied, also emotionally, for the administraton. "America is becoming vulnerable, Russia is not," Aaron told the delegates, referring to the administration's view that the Soviets will have the theoretical capability to wipe out America's land-based missiles sometime during this decade. The MX is a necessary reply, Aaron said.

"I cannot imagine the Democratic Party would repudiate the president on such an important, fundamental strategic decision," Aaron said.

To make sure that was true, Carter campaign officials joined those from the administration to lobby intensely fro support on the MX. Neverthless, at least a dozen Carter delegates joined those supporting Kennedy to vote against the missile, and more chose not to vote at all. But Smith's proposal failed by a vote of 76 1/2 to 69.

On another hotly debated foreign policy issue, the committee voted 81 1/2 to 57 against a motion to oppose draft registration in peacetime.

Early yesterday morning Carter and Kennedy aides worked out a series of compromises on other foreign policy issues.

The Carter camp agreed to a plank opposing the sale of any new "sophisticated weapons" to potentially hostile neighbors of Israel. This could be in terpreted as opposition to Saudi Arabia's request for new fuel tanks and bomb racks that could substantially enhance the American F15 fighter-bombers they were allowed to buy last year.

The Carter camp also agreed to a plank on human rights naming certain foreign countries which have been attacked by some for their records on this subject, including Argentina, Chile, Taiwan, South Korea and the Philippines.

Though Kennedy delegates and campaign officials both said they had serious disagreements with the Carter administration on foreign policy, domestic issues divided them most seriously.

Peter Edelman, Kennedy's issues director, pointed to two planks proposed from the floor in the early hours of yesterday morning and rejected by the committee as the examples of what continues to dismay the Kennedy camp.

One of the two proposals would have declared job expansion as "our single highest domestic priority" that "will take precedence over all other domestic priorties."

The second declared that the party would not support any reductions in federal funding for programs "whose purpose is to serve the basic human needs of the most needy in our society," including jobs programs, income maintenance, food stamps and education.

Both were defeated by the nearly 2-to-1 ratio Carter Delegates held over Kennedy delegates in the platform committee.

Martin Franks, a key Carter aide in the platform committee, said yesterday that the Carter troops' instructions -- presumably from the White House -- were to "win every vote." This they succeeded in doing.