MOSCOW, JUNE 26 -- The Soviet Communist Party leadership today endorsed Mikhail Gorbachev's reform drive by promoting three key supporters to the ruling Politburo, including Alexander Yakovlev, a leading proponent of the more open cultural and economic policies pursued by the Kremlin chief.

The latest promotions mean that eight of the 14 full Politburo members have been appointed since Gorbachev took power more than two years ago. Most, if not all, of those elevated under Gorbachev to the country's ruling body are considered staunch advocates of his aggressive reform policies.

Yakovlev's promotion is believed to hold particular significance because he may challenge second-ranking Politburo member Yegor Ligachev for control of the important ideology portfolio. Ligachev is thought to be more lukewarm in his backing for Gorbachev's policies, while Yakovlev has emerged as the principal architect of more modern Soviet approaches to culture and propaganda.

At the close of a major plenary meeting, the powerful Central Committee also elected to the Politburo Nikolai Slyunkov and Victor Nikonov, both economic experts whose ascendancy has coincided with Gorbachev's tenure as party chief.

It also elevated Dmitri Yazov, the new defense minister believed to have been handpicked by Gorbachev, from candidate membership in the Central Committee to nonvoting status on the Politburo, a move interpreted as a seal of approval on the ongoing shake-up of senior military officials.

The personnel moves, bringing the Politburo to 14 full and eight candidate members, appear to turn Gorbachev's bid to consolidate power at the apex of the Soviet Communist Party decisively in his favor.

In his speech today, Gorbachev signaled an intention to extend the purges he has led in the senior echelons of the party down to lower levels.

"The reform will proceed with much difficulty if we do not overcome such a difficulty as personnel reshuffles in the main section of cadres," he said, "I mean the leaders of enterprises, construction projects, collective and state farms."

Ending two days of debate described by one Gorbachev aide as "sharp," the plenum passed a series of sweeping changes in the Soviet economic structure. Both the Kremlin leader and the plenary resolution called for moving beyond debate to action.

"The political importance of the current plenum," Gorbachev said today in remarks published by the Soviet news agency Tass, "is that it translate the ideas of the reorganization into practice."

Declaring that the Soviet Union has "entered the crucial period of restructuring," the plenary resolution said, "The main thing is to build up in all ways the rates of transformation and shift the center of gravity to painstaking and purposeful organizational and ideological work."

Yet, the Central Committee's resolution of the sweeping proposals that Gorbachev made in a three-hour speech yesterday coupled ringing endorsements with a subtle note of caution.

It endorsed "radical reform of economic management," for instance, but also stressed that the plenum approves "on the whole" the draft economic resolutions and ordered the Politburo to endorse them, "with account for the discussion at the plenum."

After Gorbachev's opening speech, the discussion was heated, according to official Soviet reports.

Over 60 leading party officials spoke during the two-day session, some voicing "sharp" opinions, according to Abel Aganbegyan, a Gorbachev aide who is a leading reformist economist. "There were quite a lot of emotional speeches," he said in a news conference here tonight. "People voiced criticisms. There were disagreements. Sometimes opposite points of view were expressed."

But among influential party leaders, there was unanimous support for the reforms, Aganbegyan added.

In addition to the blueprint of guidelines for managing enterprises and firms, the plenum also endorsed a special conference of leading party officials to be held June 28, 1988.

According to the plenary resolution, the conference will be used to review the progress of the reform drive and to explore possible measures to democratize further Soviet society, but western and Soviet experts also said that Central Committee members may be put up for reelection there.

Central Committee members are usually reelected at Communist Party congresses held every five years. At the February 1986 congress, about 60 percent of the 307 members returned were holdovers from previous Soviet leaderships and have since given some of Gorbachev's proposals lukewarm blessings.

Western analysts in Moscow regard Yakovlev's election to senior Politburo status as the biggest step yet in Gorbachev's consolidation of power.

Yakovlev, 63, became Central Committee secretary in May 1985 -- three months after Gorbachev came to power -- and has charted a steady gain in influence during Gorbachev's tenure.

A major figure behind the scenes of the Geneva and Reykjavik, Iceland, U.S.-Soviet summits, Yakovlev was promoted to candidate Politburo status last January at the party plenum on personnel matters.

In the past two months, Yakovlev has launched a high-profile bid to take over the the party's ideology portfolio from Ligachev, devoting three major public appearances to ideology.

Ligachev, the party's 66-year-old chief ideologist, has used the post to deliver conservative positions on key Gorbachev-backed policies, such as the campaign for glasnost, or general openness.

Western analysts here see a clash developing, in which Ligachev may eventually assume another Politburo portfolio or limit his ideological grasp to domestic areas, while Yakovlev, who served as ambassador to Canada from 1973 to 1983, handles international ideological matters.

Nikonov, 58, has been the Central Committee secretary in charge of agricultural affairs and is likely to continue to oversee agriculture from his new Politburo post.

Slyunkov, also 58, is thought to be the new Politburo specialist on economic administration. Formerly a party leader in the Byelorussian Soviet Socialist Republic, he is thought to have close links to Prime Minister and Politburo member Nikolai Ryzhkov.

Slyunkov was brought to Moscow in March 1986 to become a nonvoting Politburo member and was made a Central Committee secretary in January.

Yazov was promoted to defense minister from deputy minister in charge of personnel last month after his Sergei Sokolov was ousted from the defense post following the landing of a young West German pilot on Red Square.

Sokolov was retired from his nonvoting seat in the Politburo today.

Dinmukhamed Kunaev, ousted last December from the Politburo, was stripped of his seat on the Central Committee in a major rebuke to the so-called old guard politicians.

The strength of the old guard circles in the ruling body who pose a potential threat to Gorbachev is uncertain.

In his speech yesterday, Gorbachev refrained from even indirect criticism of Ukrainian party boss Vladimir Shcherbitsky, 69, an ally of the late Soviet leader Leonid Brezhnev often depicted as the strongest opponent to Gorbachev in the Kremlin leadership.