MEXICO CITY - Drug violence, an economic downturn and recent cases of political malfeasance weighed heavily as Mexicans voted Sunday in midterm congressional elections that could decide the future of President Felipe Calderon's anti-crime and economic policies.

Calderon's National Action Party, PAN, hopes its nationwide crackdown on drug cartels will win it a bigger share of the 500-seat lower house of Congress, where it currently holds 206 spots.

But with the economy in its steepest downturn since the 1990s, polls suggest the gains will go to the former longtime ruling Institutional Revolutionary Party, PRI, which now has 106 seats.

"The fundamental problem is the lack of opportunities, jobs, education," said government worker Thelma Flores, 46, as she waited to cast her ballot. "That's what generates the other things, the criminality and organized crime. It's because of a lack of opportunities."

The PAN ran a bruising campaign in which it practically accused the PRI of tolerating drug trafficking. That has angered PRI members. If the party and its allies win enough seats to form a majority, they could block Calderon's efforts to reform police forces and give more police powers to 45,000 soldiers deployed to fight well-armed drug gangs.

Mexico City lawyer Jose Ignacio Ugarte, 47, said he would vote for "anybody but the PAN," a party he said had led the country to something approaching "a state of war."

The vote for 565 mayors and six governorships - including the northern border states of Nuevo Leon and Sonora - is also seen as a referendum on an economy that shrank 8.2 percent in the first quarter and is expected to contract 5.5 percent for the year as a whole.

The economic crisis has been compounded by a drop in money sent home by Mexicans working abroad and by a decrease in oil income from the slump in world petroleum prices.

Many activists and intellectuals have urged voters to annul their vote or deface their ballot in protest against the largely government-funded political parties that have done little to break Mexico out of the doldrums.

The null-vote movement wants reforms such as making recalls of elected officials easier and allowing write-in votes or independent candidates.

But few people who gathered at polling places Sunday said they were planning to annul their ballots.

Many more Mexicans - perhaps as many as 70 percent of the 77.5 million registered voters - are likely to simply stay away from the polls.

The PRI appears likely to win most statehouse races. One of the PAN's biggest hopes lies in Sonora, where the PRI state government's image suffered after a fire at an ill-equipped, government-approved day-care center killed 48 children in June.

Such tragedies, a wave of arrests of public servants and police for drug-related corruption and a string of highly publicized kidnappings and extortions have added to the disenchantment with politicians.

The leftist Democratic Revolution Party, whose candidate Andres Manuel Lopez Obrador barely lost the 2006 presidential race to Calderon, currently has 126 seats in Congress but has suffered serious internal splits. It is expected to drop precipitously after some of its more militant members turned to the smaller Labor Party.

The PRI ruled Mexico for more than seven decades until it lost the presidency in the 2000. The party has become more fractious and dominated by state leaders and regional interests since it lost national power.

Still, its extensive party machine and broad national presence would give it an edge in the event of a small turnout or a large number of protest votes.

"To the extent people nullify their ballots, institutions will be weakened and the PRI's network of control will go into action, and they will win a majority," warned the conservative, PAN-aligned civic group Better Society, Better Government.

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Associated Press Writers Nacha Cattan and Michael E. Miller contributed to this report.

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