Earlier this month, Mexican President Ernesto Zedillo, titular head of the Institutional Revolutionary Party (PRI), figuratively lopped off his index finger, or dedo.

For 70 years, chief executives had simply pointed to the PRI presidential nominee in a process known as the dedazo. Reliance on this authoritarian practice this year might have split the party, especially if Zedillo had handpicked a technocrat lacking the political savvy to deal with mounting poverty, narco-trafficking, street-crime, and small guerrilla bands in the South.Thus, the 48-year-old economist-turned-politico opened the selection of his party's standard-bearer to any of the nation's 57.5 million registered voters who cared to participate in the Nov. 7 primary.

As a result, the winner, former Government Secretary Francisco Labastida Ochoa, and party executives beam with confidence that the PRI will capture the mid-2000 general election.

Such optimism contrasts sharply with the gloom engulfing the PRI headquarters two years ago, when the party seemed to have one foot in the grave and the other on a banana peel. Although the ''PRIistas'' had ruled Mexico since 1929, they had recently lost the Mexico City mayorality, their majority in the 500-member Chamber of Deputies, and the governorships of a half-dozen key states.

Many observers assumed that the PRI would lose the presidency in 2000. They debated only whether Vicente Fox of the center-right National Action Party (PAN) or Cuauhtemoc Cardenas of the left-nationalist Democratic Revolutionary Party (PRD) would succeed Zedillo in the presidential palace.

This crepe-hanging coincided with the PRI's nominating a gubernatorial standard-bearer in Chihuahua, the huge PAN-run state to the south of Texas and New Mexico. Believing they had nothing to lose, PRI operatives abandoned the dedazo in favor of choosing the candidate in a primary.

Not only did the attractive intraparty winner in Chihuahua roll to victory against opposition candidates, but the revolutionary party proceeded to name seven other successful gubernatorial aspirants via primaries.

These feats explain Zedillo's act of self-amputation. Still, party dinosaurs groused that throwing open this critical choice would wreak havoc. They warned darkly of an embarrassingly low turnout, the inability to recruit militants to staff 64,000 voting places nationwide, and the possibility that a bare-knuckled internecine battle would rupture the faction-ridden party. The upshot, they insisted, would be a PAN or PRD triumph.

As recently as mid-October, Zedillo was urged to cancel what had wound up as an expensive, acrimonious, and freewheeling slugfest between Labastida and Tabasco's populist governor, Roberto Madrazo.

The wisdom of the president's rejecting such advice became evident Nov. 7. Despite cold weather and rain in many areas, some 10 million Mexicans went to the polls, awarding the lion's share of their votes to Labastida.

So few serious irregularities marred the balloting that even Zedillo's nemesis Madrazo, whom PRD insiders had urged to jump ship, vowed to stick with the PRI.

Madrazo has yet to accept Labastida's invitation ''to have a cup of coffee together.'' However, at age 47, the Tabasco native - a seasoned office-holder used to the rough-and-tumble of electoral politics - realizes that assisting Labastida in 2000 will enhance his own presidential chances in 2006.

Mexico's constitution limits chief executives to a single six-year term.

The primary not only boosted the 57-year-old Labastida's credibility, but it also:

* Ensured that future presidential aspirants would stress issues, popular appeal, and organizational prowess rather than skill at toadying to the incumbent.

* Furnished the PRI with both a list of millions of ''favorables'' and evidence of its strength and weakness in 300 districts from which members of congress will be elected next July.

* Buried forever the authoritarian dedazo as a selection mechanism for PRI chief executives.

One national primary did not transform the PRI into a paragon of internal democracy. Zedillo's entourage unmistakably signaled its preference for Labastida, deemed most likely to continue the nation's market-oriented reforms that spurred a 3.5 percent gain in gross domestic product this year.

The cues emboldened a majority of governors to weigh in on behalf of the president's favorite. And inadequate monitoring of lax campaign-finance limits allowed Labastida and Madrazo to spend pesos like water.

Still, in slicing off his dedo, Zedillo snatched the PRI from death's door, weakened the ability of future presidents to arbitrarily select successors, and involved almost one-fifth of the electorate in a decision that once was made by one man. Provided the party achieves unity, the process also endowed Labastida with a solid advantage over Fox and Cardenas, both of who are bedeviled by intraparty conflicts.