The illusion that ''soft-liner'' Mikhail Gorbachev needs a triumph at this week's summit with Ronald Reagan to buttress his power against Politburo ''hard-liners'' is shattered by an inside account of how his clone and erstwhile favorite, ex-Moscow Communist Party boss Boris Yeltsin, was sacked.

Yeltsin's ouster appears not to have resulted from anti-Gorbachev intrigue by a cabal headed by Yegor Ligachev, the Politburo's No. 2, whose portfolio includes party ideology. Since the Yeltsin affair, Ligachev has been portrayed by U.S. policy-makers as the antireform tough guy out to block proreform, prode'tente good guy Gorbachev.

To the contrary, a highly placed Kremlin source says Yeltsin broke a secret agreement with Gorbachev to remain silent ''until after the {Nov. 7} holiday'' about his frustration over the pace of reform. Instead, Yeltsin humiliated Gorbachev by going on the attack -- off the cuff and without notice to his patron -- for four minutes at the Oct. 21 Communist Central Committee plenum.

That undercuts a favorite illusion of de'tente-minded U.S. Kremlinologists that there is strong political differentiation inside the Politburo waiting to be exploited by Washington. They regard the Kremlin's ruling body as divided into blocs of hard-liners constantly plotting to increase their power at the expense of soft-liners.

Because the supposed soft-liners are presumably nicer to the West, they must be encouraged by U.S. policy. The Yeltsin affair was a perfect example. With Gorbachev's prote'ge' allegedly sacked by his hard-line enemies, the Soviet chief would need a little quiet assistance.

The truer account of Gorbachev's most embarrassing political crisis since he took power in early 1985 comes from one of his trusted lieutenants and is not susceptible to proof. Not surprisingly, it casts the affair in a light more favorable to Gorbachev than the accepted version, which depicts him as forced to sacrifice his closest ally. Nevertheless, the detail supplied us by a Central Committee member gives it a ring of veracity.

Last summer, Yeltsin confided to Gorbachev his growing frustration about reform in the decisively important Moscow region -- the city itself and the district around it, all under his control. This was a plea from the Soviet ruler's closest personal ally, born exactly one month before Gorbachev.

The general secretary was busy at his summer vacation hideaway working out plans for the potentially difficult Oct. 21 Central Committee meeting. He asked Yeltsin to say nothing about the matter. ''After the holidays'' (meaning the Nov. 7 anniversary of the Bolshevik Revolution), he notified Yeltsin, he would take up all his problems.

According to this account, Yeltsin agreed to say nothing. But after the Central Committee convened, he suddenly took the floor without warning. Speaking without notes for four minutes, slowly and emotionally, he portrayed himself as totally dissatisfied with the progress of reform. His attack on party cadres was implicitly an attack on party leader Gorbachev for falling down on the job.

That put Gorbachev in an untenable position. His ally had described reform as a failure in the capital of the Soviet Union and had used the sacrosanct Central Committee as his forum. He had preempted Gorbachev, who had not even admitted to other Politburo members that Yeltsin had forewarned him months before about serious trouble in Moscow.

That, we were told, placed Gorbachev at the very least in an ''ambiguous'' position from which there could only be one safe exit: bounce Yeltsin. On that point, there was unanimity between Gorbachev and the entire Politburo.

In this first real crisis of the Gorbachev era may lie seeds of big trouble for him with the Politburo. But the United States can do nothing whatever to affect the timing of this trouble or, if it comes, the course it would run. No such clash would likely be based on the fabled struggle between hards and softs. What happened to Yeltsin certainly was not.

The Yeltsin illusion is a danger for U.S. policy-makers on summit eve, but it is not the only one. White House aides also worry about what they call Secretary of State George Shultz's illusion that his close relationship with Soviet Foreign Minister Eduard Shevardnadze, and President Reagan's personal feeling for Gorbachev and regard for him as a new Soviet man, are reasons for progress on the INF nuclear treaty.

There is no evidence to support such fancies when measured against the long, embittered course of U.S.-Soviet relations. The forthcoming summit should be a good place to end illusions that depict Mikhail Gorbachev as a sweet-tempered good guy beaten down by predatory foes into sacking his closest foe. But, in fact, the meeting may dangerously enhance them.