Abolfazl Qassemi has plenty of time these days to think about what went wrong.

A slight, soft-spoken man with wispy white hair and a drooping white mustache, he is the current leader of the National Front, the political group founded by Mohammed Mossadegh that briefly ousted the shah from power in the early 1950s but lost its leadership of the opposition as a new and ultimtely successful movement against the monarchy gathered stream in 1978. a

Now Qassemi's hopes of again reviving the Front have been set back by the annulment of his recent election to Iran's Islamic parliament.

Qassemi, a former head librarian at Tehran University, makes no secret of his contempt for the Moslem clerics who he thinks are leading Iran toward dictatorship. He is one of the few political figures of any stripe to speak out publicly against the holding of the American hostages.

After Qassemi's landslide election as a parliamentary deputy from the northeastern town of Dargaz, however, a clergy-dominated Tehran newspaper charged that he had been a member of the shah's secret police, SAVAK, and printed a figure that it said was his "secret code number."

The Interior Ministry, run by Aytollah Mohammed Reza Mahdavi Kani, said it had documents proving Qassemi was a SAVAK agent, and it declared him unelected. No documents were ever produced.

The episode was one of the latest examples of a systematic campaign by Iran's powerful Moslem clergy to eliminate competition for power from remaining National Front adherents, other moderates and leftists.

Despite the Islamic republic's pretensions to free parliamentary elections, Qassemi's case shows that the authorities can always have the last word if they dislike the results.

"A certain group of reactionary elements is attempting to monopolize power in this country," Qassemi said in a recent interview in Tehran.

"During the Mossadegh era, what began as a tiny opposition in parliament mushroomed into a majority that came to power. But it appears that the present government, monopolistic and greedy and selfish as it is, simply doesn't want the tiniest of minorities, the slightest opposition. They fear that the opposition would be able to communicate with the public and receive exposure and that the people would be attracted to what it says."

Qassemi said one of his main fears is that "public dissatisfaction with the reactionary dictatorship could become so great as to provide fertile ground for the growth of communism in Iran. The organized communist groups have seemingly endless funds."

On the hostages, Qassemi said, "In my opinion the holding of hostages is not a correct or sensible thing to do. The sooner this situation is resolved the better." He called for their release, adding that "the U.S. government should be willing to give assurances it will not act directly or indirectly against the Iranian revolution."

Qassemi and his followers acknowledge that the National Front has fallen a long way since it appeared to be making a comeback at the very outset of the movement that toppled the monarchy. Most of its candidates were resoundingly defeated in the parliamentary elections, and some of its most prominent members were dropped out.

The Front's eclipse has given Qassemi time to ponder how it was overtaken by the clergy and how control of the country's destiny shifted from moderate nationalists to strict Moslem fundamentalists.

"Basically there were two reasons for this." Qassemi said. "When the ball started rolling, the clerics showed eagerness to work with us. We decided by democratic means to be faithful to the unity [of the antishah movement] and allowed the religious to join forces with us. In the first march in which we allowed them to march with us [in December 1977] I noticed a religious group tearing down posters of Dr. Mossadegh.

"In hindsight it was an error not to take that as a sign of what was coming. Repeated sgnals were taken indifferently, their significance minimized by national groups until it became to late. We innocently remained faithful to the theme of alliance."

The second reason, Qassemi said, was former prime minister Mehdi Bazargan's role in keeping the nationalist and religious elements of the opposition united behind Khomeini.

"I regret the role of Bazargan," he said. "In effect he forgot his nationalist face. He gave time and opportunity to reactionary religious elements to consolidate their position." He added: "I also was at fault. We really didn't understand the reactionary elements in this country."

Qassemi said he refuses to go underground in the face of the accusations against him and continues to cling to withering hopes that the National Front can be resurrected.

"I believe the Iranian public will become disenchanted with extreme right and left forces in the country," he said.

"Politically they are beginning to look for something in the middle. We have to organize the public and strengthen ourselves. As the situtation continues, the National Front could eventually find itself in a position in which the people call on it to govern."