Polling shows most Mexicans want an up or down vote in next July's presidential election: a decision whether to try something new or continue with the Institutional Revolutionary Party that has governed for the last 70 years.

But the choice is not going to be that simple. Because of conflicting agendas and competing egos, the vote is shaping up as a three-man contest that, following a familiar formula, could once again divide the opposition and hand victory to the ruling party, known by its Spanish initials PRI.

Negotiations aimed at forging a coalition among eight opposition parties collapsed Tuesday after sputtering for months. The collapse, which was expected, strengthened the chance that Mexico's opposition again will be too divided to defeat the PRI, which has won every presidential race since it was founded in 1929, making it the longest continuously ruling party in the world.

"The candidates' egos were the clear winners here," said Joel Estudillo, an analyst at the Mexican Institute for Political Studies, who criticized the failure to forge an alliance as a "clear sign that none of the parties is ready to assume responsibility."

On the same day the alliance talks fell apart, Cuauhtemoc Cardenas of the left-center Democratic Revolutionary Party (PRD) resigned as mayor of Mexico City to run full-time for president under his party's banner. Rosario Robles, the No. 2 official under Cardenas, was appointed to complete his term, becoming a top contender for mayor in elections next year.

Recent surveys show that two-thirds of Mexican voters favor an allied opposition to create a two-man presidential race. But the polls show that a three-man contest slightly favors whoever is the PRI nominee--former Interior Minister Francisco Labastida Ochoa or Tabasco Gov. Roberto Madrazo Pintado--over the next-ranking candidate, Vicente Fox of the center-right National Action Party, known as the PAN. Cardenas places a distant third.

Even so, a PRI victory in a three-party election is far from certain. The ruling party is in a bruising, divisive primary battle that some analysts believe threatens to divide it. And in recent months, many alliance proponents have rested their hopes for a united opposition on the prospect--however dim--that one of the two main opposition candidates will be so far behind at the end of the race that he will withdraw and endorse the other.

Attempts to forge an opposition coalition finally failed over how to chose its presidential nominee. But the talks have been faltering for months, weighed down by several factors that would have made a common front next to impossible:

* Deep political differences divide the PRD, a vaguely leftist party favorable to a strong state role in the economy and devoted to Mexico's strict separation of church and state, from the PAN, many of whose members are traditional Roman Catholics with a business-oriented outlook. It was never clear that even if party leaders could agree to unite, rank and file members would vote for the other party's candidates.

* The nearly messianic drive and egos of the two main opposition leaders, Cardenas of the PRD and Fox of the PAN, both of whom have dedicated years to becoming the first non-PRI president in modern Mexican history.

* Legal and procedural hurdles that make forging an alliance difficult and unattractive. Under Mexico's electoral law, for instance, opposition parties have to adopt a single platform and agree to one alliance candidate for every contested federal election, including for the legislature. But they could not combine campaign spending limits, giving the ruling party a substantial financial edge.

Given those and other challenges, attempts to create an alliance were problematic from the very beginning. The effort was driven chiefly by a small group of elite intellectuals and politicians who saw a coalition as the best, and perhaps only, way to beat the PRI. Despite publicly supporting the concept, it was never clear that Cardenas or Fox truly backed the idea or would support a coalition candidate other than himself.

"The two parties can't stand each other," said Soledad Loaeza, an expert on Mexico's political parties at the College of Mexico. "Both candidates have three years in the making. It was almost ridiculous to think that either of them would step down."