After 61 years of public and clandestine life, Mexico's Communist Party, one of the world's oldest, is formally dissolving today. But as the institution dismantles and scraps its name in a final party congress, it is merging with four smaller parties in a new leftist opposition front.

With this ceremony, much of Mexico's scattered left appears to accept what its more unified Central American counterparts have recognized for several years -- that squabbles over doctrine and tactics among Marxists may go on indefinitely, but only a coalition will ever provide the chance to challenge the status quo.

In contrast with the armed left in the neighboring countries, however, Mexico's Communists and Socialists have chosen the electoral route, and the new leftist coalition will run its first, and still unnamed, candidate in June's presidential election.

The Communists are expected to dominate the coalition. While giving Mexico's left a new impetus, it hardly poses a threat to mainstream politics. Mexico's government machine, known as the Institutional Revolutionary Party, has ruled since 1929, and there appears little doubt that the official candidate, Miguel de la Madrid, will become president on Dec. 1, 1982.

The sheer strength of the political establishment traditionally has discouraged other serious bids for power both on the right and the left. But the government, alarmed by a spell of guerrilla activities in the early 1970s and an ever-growing abstention rate, decided to modernize the nation's bureaucratic politics and began to encourage its opponents by offering guaranteed seats in Congress for registered political groups.

This permitted the Communist Party to come in from the cold, and four years ago it was given legal registry. It now has 19 of the 100 seats in the lower house that are reserved for minority parties. There are 300 other seats, the heavy majority of which are held by the ruling party.

Although small, the Communist Party has long been the most coherent and best organized group on the Mexican left. Since its creation in 1919 it has been forced underground on and off, but it played a crucial role in the 1930s in forming the organized labor movement. The official party, however, co-opted both the labor and peasant movements, leaving the Communist Party isolated and little more than a debating society of intellectuals. It became active again in the late 1950s.

Other leftists, regarding the Communists as dogmatic, abstruse and Moscow-oriented, founded some of the small new parties that are now participating in the coalition.

It was Arnoldo Martinez, secretary general of the party at its dissolution, who tried for the past decade to unite the left and attempted to give the Mexican Communists an image of being more pragmatic, critical and independent of Moscow along the lines of the Eurocommunists of Italy and Spain.

The party has claimed 112,000 adherents in a population of 72 million. In its first official run, in the 1979 congressional elections, it got 704,000 votes or 5.4 percent of the national total. In Mexico City alone, it officially got 14 percent of the vote.

The Communist Party and its new partners, however, may have lost their immediate chance to emerge as a significant political force last month when one important group suddenly pulled out of the interparty unity talks. Heberto Castillo, a former political prisoner, university professor and head of the Mexican Workers' Party, called the Communists intransigent because they insisted on imposing their dogma and their structure on the new coalition.

Castillo, this country's most prestigious noncommunist leftist leader, had been expected to put an appealing face on the new coalition by running as its presidential candidate. In response to Castillo's withdrawal, the Communist Party called him sectarian and bureaucratic, and said the new grouping would be formed anyway.

Government politicians, at first put off by the unity talks, now appear reassured that the leftist front will not embarrass them with a huge vote count in 1982. But one government official, who asked not to be named, said, "The truth is, we don't really know how large the left is or can be in this country. There are millions of Mexicans who don't vote. So far abstention has worked in our favor. The question is how much of that the left can pick up." withdrawal, the Communist Party called him sectarian and bureaucratic, and said the new grouping would be formed anyway.

Government politicians, at first put off by the unity talks, now appear reassured that the leftist front will not embarrass them with a huge vote count in 1982. But one government official, who asked not to be named, said, "The truth is, we don't really know how large the left is or can be in this country. There are millions of Mexicans who don't vote. So far abstention has worked in our favor. The question is how much of that the left can pick up."