MEXICO CITY, OCT. 16 -- In a move that promises to enliven an otherwise predictable election campaign, the leader of a dissident faction of Mexico's long-ruling political party has launched a rival run for the presidency.

Cuauhtemoc Cardenas, a leader of the self-styled Democratic Current of the ruling Institutional Revolutionary Party (PRI), accepted a nomination Wednesday to run in next year's presidential election as the candidate of the Authentic Party of the Mexican Revolution, a leftist splinter party that historically has supported the PRI. At least two other leftist parties reportedly are considering throwing their support to Cardenas, son of revered former president Lazaro Cardenas.

{A PRI communique said Friday that the party had expelled Cardenas, Reuter reported from Mexico City.}

While the ruling party officially dismissed Cardenas' political base as "microscopic," party sources said there is some concern that he might eventually cut into the PRI's leftist and union support.

In any case, Cardenas' candidacy is not viewed as a major threat to Budget and Planning Minister Carlos Salinas de Gortari, who has started campaigning as the ruling party's candidate after winning the nomination Oct. 4. The competition among six party leaders for that nomination is widely considered the real presidential race, although the electorate did not have a chance to vote in it.

Instead, the choice followed a Mexican tradition called tapadismo in which the incumbent president handpicks his successor. The PRI, an all-embracing political machine that relies principally on support from unions, peasants and the bureaucracy, has not lost a presidential contest since it was founded in 1929, and the election of Salinas next July for a six-year term is taken for granted.

Cardenas, an ambitious former PRI governor of Michoacan state, emerged last year as a leader of the Democratic Current, a movement within the party that upset PRI leaders by publicly demanding an end to tapadismo and adoption of more leftist economic policies including a moratorium on foreign debt payments.

Backed by another leader of the Democratic Current, former Cabinet minister Porfirio Munoz Ledo, Cardenas had sought an open presidential convention so he could run for the party's nomination against the six "precandidates" chosen by President Miguel de la Madrid. The main objective of the Democratic Current, according to observers here, was to block the candidacy of Salinas, a 39-year-old Harvard-educated economist largely responsible for implementing an economic austerity program that has incurred the wrath of organized labor.

In response to Cardenas' announcement that he would run as the Authentic Party's candidate, the PRI charged that he had renounced the party that his father helped build when he was president in the 1930s.

"I haven't betrayed my principles or the PRI," Cardenas said yesterday. "The ones who have betrayed the party are its current leaders." Munoz Ledo, insisting that he was still a member of the ruling party, called Cardenas "the real candidate of the revolution." They made the statements after participating in a march by labor and student groups to demand a halt to payments on Mexico's $103 billion foreign debt.

Cardenas also said that besides the Authentic Party, he has the support of two other small leftist groups, the Socialist Popular Party and the Socialist Workers Party.

According to Mexican economist and political commentator Jorge Castaneda, "It is clear that Cardenas and Munoz Ledo want an independent presidential candidacy that will rally the support of most of the left in the country."

In the last presidential election in 1982, five Mexican leftist parties combined received fewer votes than the conservative National Action Party, which ran second to the PRI with 16 percent of the vote, according to a debatable government count. The National Action Party is scheduled to choose its presidential candidate in a national convention next month.

Salinas, meanwhile, has begun a year-long period of extensive travel around the country until, barring the unforeseen, he takes office in December 1988. While pressing his campaign theme of modernization, he also has held several meetings with Mexico's powerful 87-year-old labor leader, Fidel Velasquez, in an apparent effort to resolve differences over economic policies that unions say have resulted in sharply reduced purchasing power.