Massoud Rajavi has a New Year's resolution to warm the heart. Rajavi is leader of the Iranian People's Mojahedin, and this year he wants to sack the Ayatollah Khomeini.

The People's Mojahedin is already the most viable opposition to Khomeini inside Iran. With growing respect from outsiders, including the United States, Rajavi and his followers are beginning to get under Khomeini's skin. Last year was a good one for the People's Mojahedin, and Khomeini and his men were forced to concede that the group was eroding his power.

In one form or another, the People's Mojahedin has been struggling against two dictatorial regimes for 22 years -- first the shah and now Khomeini. The people of the Mojahedin were robbed of the anti-shah revolution, which they may have owned more than anyone else. First allied with Khomeini, the People's Mojahedin woke up not long after Khomeini began his despotic rule.

The fight against Khomeini began in the early 1980s. The People's Mojahedin was born in a massive demonstration against Khomeini on June 20, 1981. The 500,000 marchers in Tehran were fired on by Khomeini's police, and the peaceful march became a blood bath. Khomeini foolishly gave the People's Mojahedin martyrs to fuel their anger. Since that time, the group has marked ''Martyrs Day,'' the anniversary of the slaughter, by marching in burgeoning numbers in international capitals, including Washington.

An anti-Khomeini demonstration in this country is bound to make friends, and that is exactly what Rajavi and his People's Mojahedin are doing.

In the 1970s, the People's Mojahedin could afford to be rabidly anti-American. We supported the shah, and he was imprisoning and torturing the Mojahedin's followers, including Rajavi himself. But the harassment by the shah's men paled in comparison to the savagery of Khomeini. The bulk of Khomeini's 70,000 political executions has been Mojahedin members. Another 140,000 members have been imprisoned, with some of them subjected to the most gruesome torture.

With Rajavi in exile in France, the Mojahedin continued to foment dissent inside Iran. The turning point came with an event that Khomeini masterminded to end the Mojahedin. In June 1986, the French government, in a shameless act of appeasement to Khomeini, expelled Rajavi. The same country that had coddled Khomeini in his exile before his return to Iran when the shah fell was now quaking before terrorist demands. Unless Rajavi and his followers were booted out of France, the Iranians said no French hostages would be released and more might be kidnapped.

Instead of taking his expulsion lying down, Rajavi flew to Baghdad and slowly organized several thousand Mojahedin men and women into fighting units. He called them the National Liberation Army and has used them for dozens of cross-border raids, killing, by their count, more than 4,000 Khomeini Revolutionary Guards last year. By last September, Iranian President Ali Khamenei was actually admitting publicly that more than 1,200 Iranian officials or soldiers had been killed by ''terrorists.''

Clearly the People's Mojahedin is the major anti-Khomeini force for allies and opponents of the ayatollah to reckon with.

American reckoning is taking shape in part because of the arms-for-hostages scandal. If we could bed down with the likes of the ayatollah, we could certainly let bygones be bygones with the People's Mojahedin.

Assistant Secretary of State Richard Murphy went out of his way in a July 1985 congressional appearance to label the People's Mojahedin Marxist, anti-U.S., anti-Western terrorists who had killed seven Americans.

When Murphy appeared before the same congressional committee last April, he acknowledged his earlier assessment and then said the People's Mojahedin may have changed after its break from Khomeini. He carefully said he had no evidence that the group was still anti-American. ''I don't want to overstate our knowledge of the organization,'' Murphy said. ''I will freely admit there were gaps in our knowledge about the organization.'' Murphy went on to say that the State Department was meeting with People's Mojahedin officials, having recognized them as viable ''players'' in Iran.

Rajavi has previously promised that, if he came to power, he would organize democratic elections in six months. It remains to be seen, though, whether he is sincere or just another despot looking for a kingdom.

One disturbing side note is Rajavi's articulate public relations force's painting him in the most glowing terms in Europe and the United States while Rajavi himself refuses to answer questions from reporters. He is too busy, he says, with military operations.

But Rajavi issues proclamations, press releases and telegrams from his Iraqi headquarters with a regularity that suggests his sales pitch is more important than his military operations.