López Obrador's critics have depicted the elections as a chance to stop the still-popular president from concentrating more power and weakening checks and balances. The president says the opposition is dominated by conservatives who oppose his campaign against corruption and wasteful spending.
López Obrador has complained about courts and independent regulatory agencies that have blocked some of his tougher proposals to empower state-owned industries. Opponents fear that if he wins a majority, he may try to subjugate courts and regulatory agencies created during Mexico's decades-long transition to full democracy.
As for much of the campaign, violence marked the days leading up to the vote. On Saturday, an employee of the state prosecutors' office in Chiapas who was not authorised to be quoted said five people who were carrying voting material to polling places were ambushed and killed on a rural highway.
Those killed appeared to be volunteers, not government employees. Three dozen candidates, mostly for local posts, have been killed to date.
Fifteen of the country's 32 state governorships are at stake, and all 500 seats in the lower house of Congress. Almost 20,000 local posts including mayors and town council seats are being decided in 30 states, and those have often been the most violence-scarred races. About three dozen local candidates have been gunned down.
Experts say criminal gangs have sought to influence the elections, while the government ascribes most of the killings to other questions and said they weren't necessarily related to elections.
But the country's electoral authority said the elections will be among the most thoroughly monitored in history, with over 19,000 registered observers, and violence at polling places themselves is relatively rare.
López Obrador has raised minimum wages and strengthened government aid programs like supplementary payments to the elderly, students and training programs for youths. He has also created a quasi-military National Guard and given the army a huge role in building his pet projects, which include trains, an oil refinery and airports.
But he has not hewed to a traditional leftist line. He has maintained friendly if sometimes tension-fraught relations with the United States and willingly helped keep tens of thousands of Central American migrants from reaching the U.S. border. He abhors government debt or waste.
Opponents depict him as intolerant of criticism and obsessed with a nostalgic 1960s vision of Mexico, when oil was king and state-owned companies dominated many sectors of the economy. Socially conservative and a professed Christian “in the broadest sense,” he has angered feminists with his policies, but has pleased many Mexicans by living austerely.
The elections represent the first mass public events since the coronavirus pandemic hit the country over a year ago, though case numbers have fallen and Mexico has vaccinated about a quarter of adults. The estimated 350,000 fatalities in the pandemic — about 230,000 of them test-confirmed — do not appear to have played a major role in the campaigns, but may weigh on voters'' minds.