The failure of Soviet-backed hard-liners to force Stanislaw Kania out of Poland's leadership two days ago has all but eliminated any possibility that the Kremlin can stem the tide to reform there by applying political pressure.

This is the ominious conclusion being drawn here by senior foreign diplomats and unofficial, well-informed Soviet sources assessing the fallout from the extraordinary "Kremlin letter" episode, which has brought the countries' relations to the brink of crisis.

From the beginning of the Polish unrest 10 months ago, Soviet President Leonid Brezhnev and the Soviet Politburo have been confronted by three choices: do nothing and trust the Poles will not waver from the true path; weaken any reformist movements by applying threat, bluff and direct political pressure to bolster the hard-liners; or mount an invasion.

The Kremlin leaders have pursued the middle course since last summer's strikes toppled Edward Gierek's regime. Combining the carrot of massive economic and food aid with political preachments, reproaches and constant propaganda, Moscow played for time in the hope that somehow their most loyal minions in the Polish party could gain the upper hand. This strategy may have been unworkable from the beginning, for there is plenty of evidence to suggest that the Soviet high command has not grasped the breadth and depth of reform that has swept Poland.

As the months rolled by and the Solidarity independent trade union movement gathered hundreds of thousands of Communist Party members into its membership, infusing them with the notion that fundamental democratic change was not only necessary but possible, the Kremlin's attempts to control the situation have become more shrill and exposed.

At almost every turn, the Polish leadership, by temporizing and compromising with its critics and the trade unionist-reform movement, has effectively moved beyond anything the Soviets could have foreseen when the crisis burst upon them.

Soviet tactics have sought to turn the flank of the reformers by seizing on individual incidents to spearhead a hard-line counterattack.

Last March, for example, Moscow hoped to build an incident in Bydgoszcz -- in which authorities beat Solidarity workers -- into a direct assault on Solidarity's reforms and even went so far as to have the Warsaw Pact threateningly extend its spring maneuvers in and around Poland to drive home the seriousness of Soviet interests. But the Poles resolved the crisis by heading off a general strike that seemed likely to bring on military intervention and then appointed an investigatory commission that so far has found no culprits and thus given Moscow no new openings for its anti-Solidarity drive.

Even the dispatch in April of the Kremlin's keeper of the flame of Leninist party orthodoxy, Mikhail Suslov, yielded unsatisfactory results. His session with Kania was stormy and unproductive from the Kremlin's point of view, leading Soviet news media to begin denouncing "revisionist elements" in the Polish Communist Party, echoes of which are still to be heard today.

Each time the Soviets have failed at one of these attempts, they have been forced to use sterner means of pressure. This explains the "Kremlin letter," which prompted this week's special Polish Central Committee meeting. The failure of hard-line Polish Communist Tadeusz Grabski, even with such stiff Kremlin words to back him up, to dislodge Kania is seen here as confirmation that the Soviets remain outflanked in the fast-moving Polish political arena.

Today, several Eastern European allies of the Soviet Union published the text of the letter to Poland, in what clearly was meant as a show of solidarity with the Kremlin.

The official Soviet news media have been virtually silent on the results of the Warsaw sessions but the dangerously low ebb to which relations have sunk can be gauged by the fact that the press here described Poland's top Communist simply as "Stanislaw Kania," omitting all mention of his position as party leader.

Aside from revealing probable hopes -- since gone glimmering -- that Kania was about to lose his title, the omission in the context of Soviet party symbolism was both demeaning and ominous.

Further signs of the seriousness of the situation emerged today on the domestic Russian-language service of Tass, the official Soviet news agency. In a Warsaw dispatch, Tass charged that leaflets attacking the Soviet Union and the Communist leadership of Poland were still being circulated in Warsaw despite a pledge by the Polish leadership to stop them.

The implication is that the party has again failed to live up to its promises to restore order and Moscow clearly is seeking new leverage against Kania by stressing the anti-Soviet nature of all this, asserting the leaflets contained "dirty, slanderous inventions and attacks on the U.S.S.R. and the Polish party leadership."

Through all the ups and downs of the crisis to date, most sources here have discounted the likelihood of a military move, even though three different times since last August, Soviet and Warsaw Pact troops have been brought to a state of highest readiness, which they apparently are not in now.

As Kremlin dissatisfaction with Kania grows, however, and as he and his colleagues survive one confrontation after another with the Soviets and emerge still intent upon the reform which Moscow so fears, the situation between the two countries increasingly begins to resemble that which existed between the Soviet Union and Czechoslovakia just before the 1968 invasion.

Then, the two countries' leaders were in open conflict and the Soviets, having failed to alter the situation by political pressure and maneuver, ordered in the troops. The order came four days after a sharp, denunciatory letter was sent to Prague, similar to the letter sent to Warsaw last week. It is a similarity that many observers here feel cannot be ignored.