Voters in Mexico select candidates for all 500 seats in the lower house of the national congress Sunday. President Vicente Fox and his National Action Party, known as the PAN, hope for gains, but they face stiff competition from the Institutional Revolutionary Party (PRI), the party that ruled the nation for 71 years until Mr. Fox's victory three years ago. Sunday's contest could provide an important preview of the presidential election scheduled for July 2006.

The leaders of Mexico's three main political parties are looking to Sunday's election as a way of boosting their presence on the national scene for both short-term and long-term gains. The PAN and President Fox would benefit from gains in the congress, but no one expects any dramatic shift in the legislature as a result of this election.

PRI leaders see this election as the first stage of a comeback, with the presidential contest three years from now as the real target. They have sought to galvanize their supporters nationwide with a campaign that portrays the PRI as the party of order and prosperity.

But the PAN and other parties accuse the PRI of also resorting to some of its old dirty tricks, including efforts to buy votes and to orchestrate fraud in some parts of the country. The PAN and two other parties have filed a formal complaint against the PRI with the federal Attorney General's office. The PRI denies the charges.

PAN leader Luis Felipe Bravo Mena says the PRI has failed to transform itself into a democratic and responsible opposition party.

He says the PRI has developed a political strategy based on anger and revenge, instead of developing into a responsible opposition.

For his part, PRI party leader Roberto Madrazo says the PAN and other parties attacking his party are promoting what he calls a "dirty war."

He says these tactics only result in greater absenteeism at the polls. He accuses the Fox government and even the independent electoral institute of involvement in this effort to discourage people from voting.

Mr. Madrazo is often mentioned as one of the top candidates within the PRI for the presidential nomination in 2006.

Public opinion polls show no clear winner emerging from Sunday's voting. The PAN could win a few more seats in the house of deputies, but analysts see little evidence that the party can win sufficient votes to control the congress and pass the major reforms proposed by President Fox.

The PRI, on the other hand, can maintain a strong hand by simply holding on to its current position of strength in the congress. The left-leaning Party of the Democratic Revolution is also looking for gains and a possible boost in its presidential aspirations, but internal conflicts have weakened the party.

Much will depend on which party is most successful in getting out the vote on Sunday. Polls indicate widespread voter apathy, with more than half of eligible voters planning to stay home.