Many Mexican voters see the Institutional Revolutionary Party's rambunctious presidential primary as a clash of ideas on the battlefield of democracy. But officials of the long-ruling party and political analysts say they are watching something different: a high-stakes political poker game.

The three underdogs in Sunday's vote--the first such open primary in Mexican history--have spent weeks bashing the front-runner, the party and the party's tested political machine. Repeatedly, they have claimed the electoral deck is stacked in favor of former interior minister Francisco Labastida Ochoa, the leader in opinion polls who is seen by many as the party's official candidate, blessed by President Ernesto Zedillo.

Analysts across the political spectrum say the criticisms are sincere in a hard-fought contest with high stakes; the primary could well produce Mexico's next president, who will be chosen in a general election in July. But at least as important, they say, is the message being sent by the prospective losers: Unless they get significant consolation prizes, they might refuse to endorse the winner, cry fraud or even resign from the party.

That is the ruling party's worst nightmare, because it could mean defeat. Public opinion surveys show that if such divisions split the party and its perennially faithful voters into factions, an opposition candidate could win the presidency in July, ending a 70-year reign of the longest continuously governing party in the world.

"It's a poker game, and you have to have as many aces in your hand as possible," said an independent political consultant, adding that the losers likely will file thousands of complaints of voting irregularities to give themselves bargaining chips.

The goal of the party, known commonly by its Spanish-language initials PRI, is to get all four primary candidates to stand arm-in-arm and call for party unity. If the PRI holds together and runs a clean primary, many analysts think it would be unbeatable in the general election.

The four candidates are Labastida, who is ahead at least 12 points in polls, former Tabasco state governor Roberto Madrazo, former Puebla state governor Manuel Bartlett and former party president Manuel Roque Villanueva.

Although the party itself is organizing the primary, any eligible Mexican can vote. The outcome will be based on a tally of who wins in each of Mexico's 300 congressional districts. One independent survey found that Labastida will probably win about 210 districts to Madrazo's 90.

Preliminary results are expected early Monday. The PRI nominee will be officially announced Nov. 18, allowing 10 days for losers to challenge the vote and negotiate a deal in exchange for endorsing the winner.

The two main opposition parties, the center-right National Action Party, or PAN, and the left-center Democratic Revolutionary Party, or PRD, chose their presidential nominees in primaries that did little more than endorse candidates who had sewn up nominations long ago. They are former Guanajuato state governor Vicente Fox for the PAN and former Mexico City mayor Cuauhtemoc Cardenas for the PRD.

The ruling party's precedent-setting presidential primary was orchestrated by Zedillo, who in the face of the party's declining popularity and calls for greater democracy abandoned a long-standing tradition in which the sitting president selected the party's nominee. Because of the party's long string of victories, the practice amounted to the president handpicking his successor.

The electoral reforms ushered in a freewheeling primary campaign, punctuated by unprecedented television advertising and imported U.S. pollsters and consultants who helped fuel the most negative campaign in modern Mexican history. In the final weeks, the candidates launched particularly aggressive attacks on Labastida and the party hierarchy. The main objective, analysts said, was to make clear that acquiescence in the primary should not be taken for granted, that widespread fraud would not be tolerated and that they expect to be taken care of after the primary.

Blocs of seats in Mexico's Congress are awarded to political parties based on the proportion of the vote they receive in national elections, and Bartlett and Roque may have to be offered Senate seats. Bartlett might also be interested in the position of ambassador to France, where he was a student, while Roque could be tempted with a cabinet position.

The big problem is Madrazo, who has aired relentless personal attacks against Labastida--calling him a failure and a liar, depicting him with his nose growing Pinocchio-style and portraying him as a puppet in the arms of disgraced former president Carlos Salinas de Gortari.

Madrazo has drawn a wide following with those attacks and could inflict significant damage on Labastida's prospects next July if he and his supporters bolt from the party. Observers here said Madrazo may have to be offered a sizable carrot--numerous seats in both houses of Congress for his allies, a Senate seat for himself and leadership of the PRI's congressional delegation--possibly even the presidency of the party.

"It's clear there is a possibility he could leave the party, and that's one reason Labastida is compelled to negotiate with him if he wins on Sunday," said political analyst Sergio Sarmiento.

CAPTION: Francisco Labastida is the favorite in the first primary of Mexico's ruling Institutional Revolutionary Party.

CAPTION: Cuauhtemoc Cardenas, candidate of the Democratic Revolutionary Party, greets supporters in San Cristobal de las Casas in the southern state of Chiapas.