MEXICO CITY -- Barely three months after winning a trade pact that was supposed to catapult his country into the 21st century, the regime of President Carlos Salinas de Gortari looked more like a devastated Humpty-Dumpty last week.

With the stunning peasant revolt in Chiapas state still unsettled, the candidate of the ruling Institutional Revolutionary Party's (PRI) for president was assassinated at a rally in a poor section of Tijuana on Wednesday night.

Many wondered if the momentum toward progress could be put together again, or whether, in choosing a successor to the assassinated Luis Donaldo Colosio Murrieta, the old guard would prevail with its old undemocratic inclinations.

The moment seemed chaotic.

As the nation mourned Mr. Colosio and his party prepared for the choice of a successor, optimists were hard to find.

"All the forces that have worked to build peace and to open ways for greater participation of all people are now blocked and confused," said Francisco Escobedo, a member of the PRI who has hardly had the energy to leave his house since Mr. Colosio's assassination. "All of Mexico is not only asking why, but what is to become of this country?

Coming as it did on the heels of the bloody guerrilla rebellion in January in the southern state of Chiapas, the assassination "has made the climate of insecurity intolerable," Mr. Escobedo said.

Also shattered by the assassin's bullets was the ruling PRI. Mr. Colosio was a candidate who unified the various sectors of the PRI. He was an affable man who rose through the ranks of the PRI and had made many friends among the conservative old guard. But raised on a modest ranch, the 44-year-old also understood the desires of the burgeoning middle class for greater social and political equity.

New candidate needed

The PRI has only five months to select a new candidate who will win the support of both camps within the PRI, organize a national campaign and win an election that is expected to be the most scrutinized in the history of the country.

Fraud, widely practiced to keep the PRI in control of the Mexican government for more than 60 years, is hardly an option because of new laws that give citizen groups more say in the way elections are organized and the facilities to monitor the electoral process.

"There is a new reality in Mexico, and that is that the PRI candidate is not automatically going to win the election," says Sergio Aguayo, a political analyst at the Colegio de Mexico. "It was like a monarchy, but that has changed. This is the year of transition and change."

In a stunning speech early this month, in honor of the 65th anniversary of the PRI, Mr. Colosio spoke forcefully about this change.

"Today we face authentic competition. The government will not give us triumph. The triumph will come from our work, from our forces, from our dedication," he said. "Today, before the PRI of Mexico, before all Mexicans, I express my commitment to reform power, to democratize it and to do away with any vestige of authoritarianism."

Those words still ring in the minds of Colosio supporters, like Mr. Escobedo, who see the candidate's murder as an attempt to snuff out the weak flame of democracy.

"Colosio was becoming committed to social change. He was becoming a threat for those who profit from the status quo," said Primitivo Rodriguez, of the Mexican Academy for Human Rights. "Now Mexico is facing an unprecedented choice: the option for democracy or the option for more centralism and authoritarianism, repression and abuse."

Fear of regression

Mr. Escobedo, who calls Mr. Colosio a martyr, says he fears that the old-line PRI forces will use the assassination of Mr. Colosio and the Chiapas rebellion to force the party to halt liberal policies.

"One of the greatest risks is that the more impermeable forces in the party will emerge and say, 'You see what happens when changes occur too quickly? You see how dangerous it is? What we need are more strict measures,' " he says.

In a letter yesterday to the people of Mexico, the Zapatista rebels of Chiapas grasped Mr. Colosio's promises to compete fairly against other political parties. Spokesman Subcommander Marcos expressed outrage that "the hard line and military option within the federal government brought about this provocation to annul all pacific intents of democratization of national political life."

Because of Chiapas, the most violent manifestation of the frustration felt by most Mexicans, political analysts say that President Salinas will not be able to anoint the PRI candidate as he did when Mr. Colosio was named.

In a tradition called "the destape," or the unveiling, the president chooses the PRI candidate. Because the PRI always won presidential elections, the destape allows a president to select his successor.

In choosing a candidate to replace Mr. Colosio, many members of the PRI and political analysts say that President Salinas has begun consulting with party chiefs and leaders of Mr. Colosio's campaign.

"The president has to do some serious talking to people, and he has to listen to their suggestions," says Roderic Ai Camp, a Latin America expert at Tulane University. "He has to find a candidate who is liked by one camp of the party and at least tolerated by the other. If he doesn't, then the party will remain divided, and they are lost."

Several contenders

Several names have been mentioned as possible candidates. The favored appears to be Ernesto Zedillo, former secretary of education who coordinated Mr. Colosio's campaign. Other names mentioned include Fernando Ortiz Arana, chairman of the PRI, and Fernando Guttierrez Barrios, former interior minister.

"Mr. Ortiz and Mr. Guttierrez, would show that the PRI is looking backward," Mr. Escobedo says. "Mr. Zedillo would show that the party is looking forward."

Manuel Camacho Solis, the government's peace negotiator in Chiapas, would be a favorite to win the nomination. But he darkened his image when he publicly flirted with the thought of challenging Mr. Colosio for the presidency before the assassination.

The day before Mr. Colosio was killed, he announced he would not run for president under any circumstances. But he never endorsed Mr. Colosio. When Mr. Camacho visited the funeral home last Thursday to pay his last respects to Mr. Colosio, an angry crowd yelled, "Colosio, Si. Camacho, No."

Government officials say that the name of the new candidate could be announced as early as today or at the end of next week. Whoever is chosen, says a government official, will likely win support simply out of sympathy for Mr. Colosio's murder.

And many people, frantic that their once stable nation is falling into chaos, may turn to the PRI because it has the most proven record of maintaining order.

"This is the kind of thing that can rally people," the government official said, referring to Mr. Colosio's assassination. "Let's hope it has that kind of affect."