The death of one-party rule in Mexico promised a new era of cleaner elections.

But two studies suggest that the first presidential contest since Vicente Fox ended the Institutional Revolutionary Party's seven-decade hold on power in 2000 may be tainted by many of the same coercive tactics that marred previous balloting.

Millions of poor Mexicans have been threatened with exclusion from health care and social assistance programs if they do not vote for various candidates, the studies show. Others, mostly in rural areas, have been given cash payoffs of $40 to $60 for their votes, a tidy sum in a country where the poorest families subsist on less than $4 a day.

The authors of the studies said the coercion is so pervasive that it could swing the outcome of the July 2 election, particularly if the front-runners, Felipe Calder?n of Fox's National Action Party and Andr?s Manuel L?pez Obrador of the Democratic Revolutionary Party, finish within two percentage points of each other.

"It is ingrained in our political culture -- they sell their votes because they don't think their votes make a difference," said Sergio Aguayo, a Mexico City political analyst who oversaw one of the studies.

Democracy in Mexico "is weak," Aguayo's report said, "because beneficiaries of social programs don't know their rights and don't know how to defend themselves" when pressured to vote for a candidate.

The studies were conducted by Alianza Civica, a nonpartisan citizens' group that receives financial support from the United Nations and works to aid Mexico's transition to democracy, and the Center for Higher Study of Social Anthropology, a research institution that was hired by Mexico's social development agency to conduct independent voter research.

The organizations interviewed thousands of voters. The Center for Higher Study of Social Anthropology study included a poll of more than 4,600 voters in four Mexican states, while the Alianza Civica probe was undertaken in 101 cities in 22 states.

Electoral officials, who have conducted voter education campaigns throughout the country, said they were confident that the election would be fair but acknowledged that some vote-buying was taking place.

Marco G?mez, a member of the governing board of Mexico's Federal Electoral Institute, which enforces campaign law and tabulates results, said he did not believe the vote-buying schemes could sway the election.

"Every time we vote, these tactics are less effective," he said.

The studies said that all three major parties pressure and pay off voters. Each of the studies found that the Institutional Revolutionary Party, or PRI, is the most coercive, followed by the National Action Party, whose standard-bearers are Fox and Calder?n. Lopez Obrador's Democratic Revolutionary Party, which is the smallest and youngest of the three major parties, is the least coercive, the studies showed.

As many as 4 million voters, or about 1 in 20 of those registered, have been paid to vote for a candidate, Alianza Civica said in its report, which was released last week. Much of the coercion, it concluded, is applied by local and state officials who control health, welfare and farm aid programs. The group's director, Silvia Alonso, also faulted Fox's government for not taking a more aggressive stand against coercive practices.

"It's sad," she said. "We're still in the process of transitioning to democracy."

The authors of both studies acknowledged that Mexico had come a long way from the heyday of the PRI era, when soldiers and police often harassed voters and vote-counting fraud was common. As recently as 1988, independent observers have said, the PRI blatantly stole a presidential election by staging a fake computer crash.

"Certainly things are better now than in the past -- in the past, the president and the PRI controlled everything: the media, the electoral authority, the electoral process," Aguayo said. "But things are still not as they should be."

Half of the 4,400 voters interviewed by Alianza Civica said social programs in their areas were being used to benefit supporters of political parties. The study's findings indicate that vote-buying and coercion are likely to increase this week, in the final days before the election.

There has been almost no effort to prosecute public officials for vote-buying or coercion. The Center for Higher Study of Social Anthropology, which last week re-released a report it first presented in April, recommended increasing voter education and referring cases of vote-buying to law enforcement authorities.

"In a democracy," Aguayo said, "you have to fight for democracy every day."