The "videogate" scandal, blamed by Mr Cardenas on the ruling Institutional Revolutionary Party (PRI), has raised the temperature in the run-up to historic 6 July elections which could see the PRI lose Mexico City and control of Congress for the first time in its 68-year history.
The populist Mr Cardenas, 63, is way ahead in the race for Mayor of the capital - the second-most influential post in the country after President - as candidate for the Social Democratic Party of the Democratic Revolution (PRD). Mexicans will also elect a new 500-seat lower house of parliament, a quarter of the Senate and six state governors.
Tens of thousands of videotapes, in which images of violence and revolution were juxtaposed with doctored speeches by Mr Cardenas, were distributed to businessmen over the past few weeks until police last week raided a Mexico City apartment where the videos were being produced. The idea appeared to be to scare voters and investors in an attempt to slash Mr Cardenas's lead.
Fourteen people were arrested, but police are still investigating whether the tapes were linked to a political party. Mr Cardenas pointed the finger at the PRI, accusing them of launching a "dirty war" to avoid defeat, but the ruling party denied involvement.
In the mayoral race, Mr Cardenas, son of revered former Mexican president Lazaro Cardenas, is running ahead of Carlos Castillo Peraza of the conservative National Action Party (PAN) and the PRI's Alfredo Del Mazo, a former energy minister.
The PRI's control of the Senate is not in danger but it could lose at least one governorship, in the key industrial state of Nuevo Leon, to the PAN.
The PRI maintained control of all 32 states for the 60 years after its creation in 1929. But electoral reforms forced on former President Carlos Salinas de Gortari and his successor Ernesto Zedillo have since allowed the PAN to grab four state governorships. The left-of-centre opposition has never won a state.
While a victory by Mr Cardenas in Mexico City would be historic, some analysts say the city vote is something of a sideshow. In recent years the party has gradually jettisoned segments of power but continued to reign over Mexico's complex socio-political system through its patronising control over all sectors of society, from the police, military and judiciary to trade unions and peasant groups.
Even a loss of its parliamentary majority would not rob the PRI of its control of "the system", analysts say, although it would seriously handcuff the President in such areas as pushing through the budget, long the prerogative of the ruling party.
To offset the fears of businessmen and investors, Mr Cardenas has played down his populist image and dropped his opposition to the North American Free Trade Agreement (Nafta) with the US and Canada. He is expected to run for President of Mexico in 2000, when Mr Zedillo's six-year term expires.
Mr Cardenas's surge in popularity in the capital and the poverty-stricken south is seen partly as a protest vote against the PRI, partly as a backlash from his unsuccessful run for the presidency in 1988. That year, Mr Cardenas was running ahead of PRI candidate Carlos Salinas de Gortari when the PRI-controlled Electoral Council announced that the computer system counting the votes had crashed. When it came back up, Mr Salinas was ahead and won narrowly. Ballots were quickly burned before a recount could be made.
Mr Zedillo has since presided over electoral reforms, including a more independent Electoral Council, but the opposition warn that PRI militants may resort to the traditional fraudulent tactics, particularly in rural areas, such as the "taco" vote - rolling up several previously-marked ballot slips to look like one.
Another old PRI favourite was the use of the "dead man's vote" when electoral registers included the names of dead people who voted - naturally - for the PRI. New photo credentials should make that impossible.