Mexico has no vice president, but in next year's election, presidential candidates for the first time will have an unofficial running mate--their party's nominee for mayor of Mexico City.

Next summer for the first time, voters will elect both the country's president and the mayor of Mexico's most populous city on the same day. That means the fortunes of presidential candidates may rise and fall on the strength of their parties' candidates for mayor.

"How the couples compete together could make a humungous difference," said Luis Rubio, director of Mexico City's Center for Development Research.

With so much at stake, the mayoral election is drawing the interest of some of the biggest names in Mexican politics, and today the first of them officially announced. Jesus Silva Herzog, a charismatic career bureaucrat, former ambassador to the United States and onetime finance minister, said he is seeking the nomination of the ruling Institutional Revolutionary Party (PRI).

"I will always speak the truth and be realistic; I will not create false oases of hope," Silva Herzog said to an enthusiastic group of several hundred supporters, some wearing "Jesus 2000" pins on their lapels. "We have lost a lot of things in this city--our security, the credibility of our institutions, confidence in ourselves and in our future. We need to win this back."

The mayor's race has taken on extraordinary importance for several reasons. As the leader of Mexico's largest metropolis--more than 20 percent of the county's 100 million people live in the capital and its sprawling suburbs--the mayor occupies a high-profile platform and can have a major impact on national politics and policy.

But even more important is the potential impact of the mayor's race on the presidential contest. Political analysts expect the presidential race that culminates on July 2, 2000 to be one of the most hotly contested ever, with opposition parties having a good chance of breaking the PRI's 70-year lock on the office.

With elections for mayor and president taking place on the same day, and with what political analysts' say is Mexicans' historic ambivalence about splitting their votes, the candidate they support in the mayoral election could be decisive in a tight presidential contest--and vice versa.

About 11 percent of the electorate lives in the capital, and another 15 percent lives in the sprawling suburbs. The latter voters, while unable to cast ballots for mayor, are nonetheless in the same media market and will be bombarded by television and radio ads trumpeting and lambasting the candidates and their parties.

The July 2 ballot will be only the second in which the mayor of this city--who used to be appointed by the president--will be elected. Cuauhtemoc Cardenas of the left-center Democratic Revolutionary Party won the first contest, in July 1997. His term was set at three years to bring the mayoral and presidential races into the same cycle, but the next mayor, like the president, will serve a six-year term.

Cardenas, who is running for president in 2000, has had an undistinguished term as mayor, according to most analysts, and polls show his approval rating plummeting, dragged down principally by an inability to curb a crime wave. As a result, the other two parties--the PRI and the right-center National Action Party--see an opportunity to win the mayor's office and boost their presidential candidate at the same time.

That, according to some analysts, is what the PRI hierarchy is banking on with the candidacy of Silva Herzog, 64, a strong-willed, Yale-educated economist. Some analysts say Silva Herzog is the preferred mayoral nominee of Fransico Labastida Ochoa, the somewhat drab former interior minister who currently is favored to win the ruling party's Nov. 7 presidential primary.

Spokesmen for the two opposition parties said they expect to announce their parties' mayoral selection procedures next month.