By William Branigin
Washington Post Foreign Service
Friday, August 5, 1988; Page A25

MEXICO CITY, AUG. 4 -- Three opposition presidential candidates of the left and right today jointly rejected the official results of Mexico's July 6 elections and called for a stepped-up protest campaign to prevent the government candidate from taking office.

In a joint statement issued at a press conference, Cuauhtemoc Cardenas, Manuel Clouthier and Rosario Ibarra de Piedra said Carlos Salinas de Gortari, the candidate of the long-ruling Institutional Revolutionary Party (PRI), "did not obtain even a plurality of the votes actually cast July 6.

"From all evidence, the Mexican people voted for change, rejected the system of the official party and opted in the majority for the candidates of the opposition," the statement said. It said the three would recognize neither the results nor the authorities that "arise from fraudulent elections."

The joint statement and calls for new protests indicated that, a month after the most hotly contested national elections in Mexico this century, the political situation is still in flux and the prospects for a smooth accession by Salinas are still uncertain.

A week after the election, the Federal Electoral Commission announced that Salinas, a 40-year-old technocrat and former budget and planning secretary, had won with 50.36 percent of the valid votes, the lowest total for a PRI presidential candidate in the party's 59-year history.

The new chief executive is scheduled to take over from outgoing President Miguel de la Madrid Dec. 1.

In their efforts to prevent that, the three opposition contenders called on their followers to take to the streets Aug. 14 in simultaneous demonstrations nationwide to demand that the "popular will" be heeded by Mexico's Electoral College.

This body, which convenes Aug. 15 for the rest of the month, is to settle finally the numerous complaints of vote fraud in the presidential and congressional elections so that the winners can be formally proclaimed.

While claiming jointly that Salinas did not win, the three opposition leaders reached no consensus on who did. Their statement, in fact, underscored the differences that so far have prevented them from combining their forces at the national level and reaching common positions on the major election issues.

Cardenas, who broke with the PRI last year to run as the candidate of a leftist coalition, called for a "national mobilization" of simultaneous street demonstrations Aug. 14. A former PRI state governor and son of popular 1930s president Lazaro Cardenas, the 54-year-old populist has said he was cheated of victory when the Electoral Commission last month gave him only 31 percent of the vote in its final count.

Clouthier, also 54, the candidate of the conservative National Action Party, said his followers separately would stage a silent march Aug. 14 to the Interior Secretariat but would not participate in joint protests with the left, nor back Cardenas' victory claim.

"What we're seeking is democracy," said Clouthier, a wealthy farmer and businessman from northern Mexico. "We're not looking for an alliance of ideologies or candidates."

Clouthier, who received 17 percent of the vote in the official tally, charged that the elections were so tainted by irregularities that no presidential winner could be declared. Therefore, he said, the elections should be annulled and new ones held.

Supported by Trotskyite candidate Ibarra, who received a tiny fraction of the vote, Cardenas said he had won a "clear majority" of the presidential vote. However, he said he favored exhausting the remaining legal remedies for vote fraud through the Electoral College before joining Clouthier's call for nullification and new elections.

Cardenas, who has been drawing large crowds during a post-electoral nationwide tour, charged that the PRI-dominated electoral commission has illegally tried to disguise massive fraud by failing to provide a breakdown of the votes from 25,000 of the country's more than 54,000 voting centers. His four-party coalition wants to check the breakdown against the centers' original tally sheets.

Officials of the PRI deny more than "minor" irregularities and insist that, on the whole, the elections were the cleanest in Mexican history.

In the news conference that followed the signing of the joint declaration, the opposition candidates denied government assertions that their protest movement is losing steam.

"This government is hoping that the people calm down," said Clouthier. "But the people are not calming down."

Added Ibarra, "We think the movement is growing."

The three also agreed on a joint protest letter to be sent to the numerous foreign governments and heads of state who have congratulated Salinas.

"The Mexican electoral process has not yet ended," the letter says, adding that such congratulations constitute "interference in the internal affairs of Mexico."

Among those who have sent congratulations to Salinas are President Reagan, Soviet leader Mikhail Gorbachev, Cuban President Fidel Castro and a number of other leaders from Latin America and Europe.