The PRI, which has held the presidency and controlled congress since the party was founded in 1929, was in danger of losing its parliamentary majority for the first time as voters chose 500 new MPs. No single opposition party has the support to take over that majority but together the two main opposition parties could win enough votes to present President Ernesto Zedillo with a hostile chamber of deputies for the first time.
With only 32 new senate seats at stake - to create a larger senate of 128 seats - the PRI's control was not in danger but the party also faced close races in at least two of six state governorships at stake. Both houses have traditionally been rubber stamps for the president and successive PRI governments.
Perhaps more important, symbolically, was the race for mayor of Mexico City, one of the world's largest cities with a population of more than 20 million, which the ruling party seemed bound to lose for the first time in the PRI's history.
Under pressure for democratic reform, President Zedillo was forced to hold mayoral elections for the first time in 70 years. Previously, the president - always from the PRI for the past seven decades - handpicked one of his favourites for the job.
Cuauhtemoc Cardenas, a social democrat and 63-year-old son of the Thirties President Lazaro Cardenas, looked almost certain to become mayor by defeating both the PRI and the conservative National Action Party (PAN) candidates. Mr Cardenas was widely thought to have won the presidency in 1988 but believes he was robbed through a computer fraud which gave a narrow victory to the PRI's Carlos Salinas de Gortari.
Mr Cardenas sees the mayor's job as a stepping stone for another run at the presidency in 2000 when Mr Zedillo's six-year term expires. That means he would be mayor for only 20 months - from inauguration on 5 December - since he would have to stand down to campaign for the top job.
While his popularity has soared in the capital, Mr Cardenas's Party of the Democratic Revolution (PRD) was likely to trail a distant third in yesterday's parliamentary and state governorship votes.
Despite Mexico's tradition of fraud, most people feel Mr Zedillo has done his best to create clean elections - perhaps for the first time - through electoral reforms. But Mexican and foreign observers spread out across the nation's ballot stations, which number more than 100,000, to watch for ballot stuffing or coercion.Reuse content