Iran's radical left has jumped on the current anti-American bandwagon in a bid to end three months of political purgatory and rejoin the Iranian mainstream.

Denounced by Ayatollah Ruhollah Khomeini as countertevolutionaries last summer, the radical left went underground. It emerged only weeks after the now month-old seizure of the U.S. Embassy.

Marxist Fedayan guerrillas staged two marches in Tehran despite criticism from Islamic groups who consider them godless adversaries little better than the "Great Satan," the United States.

More indicative of their political impact was Khomeini's quick reaction to the national call to arms by the leftist Islamic Mujahideen guerrillas, who offered to provide the public with military training.

Aware of the Mujahideen's discipline, armed strength and large public following, Khomeini within a day announced his own mobilization and called for an army of "20 million" to oppose the United States.

Pro-Khomeini Islamic groups quickly started weapons training in schools, factories and on television where instructors showed how to clean, field strip and fire the standard assault rifle.

The Mujahideen owen much of their impact to their unquestioned religious credentials, which make them acceptable -- if sometimes only barely -- to Khomeini's followers in contrast to the Marxists, Maoists, Trotskyites and other atheist splinter groups.

They have kept at least part of a foot inside Khomeini's camp while also criticizing many of the ayatollah's policies.

The Fedayan, normally somewhat suspect in this militantly Islamic society because of their Marxist affiliations; are thought to have lost considerable numbers of sympathizers during the last few months. In addition, the Fedayan had weakened themselves technically by taking up arms against the central government in Kurdistan and, until recently, openly denouncing Khomeini's "anti-imperialist" crusade as a "farce."

Thus for the Mujahideen, Fedayan and other smaller leftist groups, the anti-American campaign provided the first respectable pretext to resurface.

While the two largest radical leftist groups were reemerging from their ghettos, the pro-Moscow Tudeh Communist Party continued to echo Khomeini's line on all but the most insifnificant points.

It degerential support for the ayatollah has made the Tudeh the laughing stock of politically conscious Iranians. But it also produced immediate dividends.

The once strident anti-Soviet campaign in the press, radio and television, which used to attack the Soviets intervention in neighboring Afghanistan, stopped nearly two weeks ago.

The controlled media have played down Soviet support for the U.S. complaints about Iranian violation of diplomatic immunity on the hostages question.

For example, the same day the Soviets sided with the United States on that point in the United Nations Security Council, Tudeh central committee members brought flowers to the students holding the hostages at the U.S. Embassy.

As transparent as such tactics may be, analysts here credit Tudeh with enlarging its audience. But that constituency remains limited because of the party's 40-year record of unbroken obedience to Moscow as well as bitter memories here of the Soviet occupation during World War II and after.

Tudeh is credited with becoming second only to the Mujahideen in political influence in the leftist constellation.

But that position is due to a domestic policy line aping Khomeini.

Without any known military arm, Tudeh has become a respectable part of Iran's revolutionary establishment, according to analysts. Its publications appear without difficulty. Its leaders are received in government offices and in the holy city of Qom by Khomeini's representatives, if not yet by the ayatollah himself.

The Tudeh, the only leftist party to urge a "yes" vote in the constitutional referendum, that began today, is clearly calculating that its attitude may curry favor with the 79-year-old Khomeini, who has made the vote a point of personal honor.

What lies ahead for the various leftwing groups is difficult to predict. None seems likely to try to take over the country as long as Khomeini is alive.

In fact, nothing prevents Khomeini and his religious followers from renewing attacks on the left and seeking to isolate it again.

But as the opposition to his hand-tailored constitution on the left, right and center indicates, Khomeini has failed to make the organized parties knuckle under to his conditions.

The radical left, especially the guerrilla groups, remains organized, disciplined and armed to the point of saturation -- thanks to their looting of government arsenals in the February revelution.

For the time being the leftist radicals and Tudeh are taking advantages of the obvious inability of the clergy to administer the country to improve their own positions.And if the situation again turns against them, they have a long history of clandestine operations.

Strong traditionally in the Caspian Sea area and Tehran, perhaps less so in oil-rich Khuzestan, the left has also made inroads in Kurdistan. There, in the western province demanding autonomy, the Fedayan, and even more so that Maoist revolutionary organization of Kurdish toilers, or komala, have staked out real leadership claims.

The worsening economic situation unemployment and the anti-American campaign all provide the left with traditional openings to exploit and an undreamed-of cover to pursue its aims.