The Soviet invasion of Afghanistan has capped a series of crises undermining the Palestine Liberation Organization's once promising diplomatic offensive to win respectability, stature and influence in the West.

With the Soviets' Afghan intervention angering much of the Third World, especially Islamic, countries the PLO feels deprived of its normally effective Soviet leverage to counter balance American influence in the Middle East.

Also contributing to PLO worries have been the effect of the U.S. Embassy crisis in Tehran involving its controversal ally Iran and the embarassing pulic quarrel with Libyan leader Muammar Quaddafi.

Discussions with PLO insiders here reflect disappointment and the realization that the doplomatic offensive at best is marooned during the American presidential election year.

The PLO's strategy last year was designed to avoid just that quadrennial pitfall, which Israel over the years has so skillfully exploited to justify the Middle East status quo.

The implied threat of a Middle East oil embargo was never stated, but was nonetheless a real vehicle for the offensive in favor of a negotiated Middle East settlement.

Last summer and fall PLO leader Yasser Arafat was received by officials and some heads of government in Western Europe in a strikingly effective demonstration of the new diplomatic approaches.

But now one PLO analyst has been reduced to arguing privately, and without much conviction, that "we can only hope the Soviets will get out of Afghanistan as fast as possible." He said his greatest fear was that "we are now basically defenseless before unfettered American force in the Middle East."

"Egypt is out of the military equation, and Israel is stronger militarily than ever before," another PLO analyst lamented. "Who is going to protect Jordan, Syria and the PLO with the Soviet tarred with the Afghan invasion? The Americans can walk all over us."

Paradoxically, it is basically the conservative Arabs of the oil-rich Arabian Peninsula who question this gloomy analysis of the confrontation states Jordan, Syria and the PLO have either remained silent or expressed understanding, if not approval, for the Kremlin's action. Ayatollah Ruhollah Khomeini's Islamic revolution in Iran, these conservative governments are obsessed by fears -- which they see justified by U.S. helplessness in Iran and Afghanistan -- that the United States is no longer a reliable ally.

While the PLO privately insists it is bogged down, the conservatives publicly claim the only way to save their own and Wwestern interests is through American pressures on Israel to bring about a Palestinian state in the West Bank and the Gaza Strip.

American failure to act forcefully on that score, conservatives argue, sooner or later will strengthen the Soviet influence in the Middle East.

What today is denounced as Soviet agression against a fellow Islamic country in Afghanistan, they fear, could tomorrow be hailed as an effective, if brutal, means of achieving the Krendin's aim as the Palestinians' longing for a homeland.

Palestinian moderates, hopeful that the diplomatic offensive would pay off now, regretfully note that both Israel and Palestinian radicals opposed to any negotiated settlement are never happier than during periods of U.S. Soviet tensions.

Radical guerrilla groups such as George habash's Popular Front for Liberation of Palestine and Nayef Hawatmeh's Popular Democratic Front endorsed the recent Soviet invasion of Afghanistan in a calculated effort to force an embarrassed Arafat to stop sitting on the fence and follow suit. PLO's diplomatic offensive also has embarrassed Arafat.

Fatah, Arafat's power base representing about 80 percent of the Palestinian guerrilla movement is reassessing its moderate policy. Some analyst feel it may indeed back the Soviets, but in the vaguest possible fashion.

Reflecting the anguish of Palestinian moderates over the Soviets' Afghan adventure, one PLO analyst said, "Don't get us wrong. We don't like to see any country lose its independence. That's what the Soviets are doing -- after all, trying to win back our own is what we've been fighting for for more than 30 years."