Pre-election poll showed that Mexico's opposition Institutional Revolution Party (PRI) expanded its lead by 0.3 percentage points from May to 37.4 percent ahead of Sunday's mid-term congressional elections, survey firm Consulta Mitofsky said on Tuesday.

This could mean the party, which ruled Mexico from 1929 to 2000, could win 234 seats in the 500-seat lower congress, the Chamber of Deputies, up from 104 seats now.

However, gaining the majority in the congress does not promise the party's presidency, which will be decided in the 2012 presidential elections.

The ruling National Action Party (PAN) is running second with a support rate of 32.2 percent, down 0.8 percentage points from last month. That would give it a maximum of 177 seats, down from 206 seats at present.

The Revolutionary Democratic Party (PRD), currently the legislature's second largest with 123 seats, is in the third place with 14.7 percent, down 1.8 percent. This would give it no more than 92 seats, making it the smallest of the nation's major parties.

"The PAN will lose first place in the legislature, but will do better than six years ago and reaching 167 seats means it will have a third of the chamber," said Mitofsky.

In 2003, the PAN was reduced to the third place even though it had the presidency, causing three years of legislative stalemate whereby the president vetoed legislation passed by the Congress and the Congress refused to pass legislation promoted by the president.

Polls showed that three small parties, which currently have altogether 31 seats, might end up with none at all.

High-profile figures are backing a campaign for a null or blank vote to show dissatisfaction with the party system.

Mexican elections usually generate around 2.5 percent of null votes, a figure that is normally attributed to ignorance of how the voting system works. The null vote campaign might have been considered to have an effect if it generates a null vote of more than 5 percent.

Voters will elect 300 deputies in a first-past-the-post system at 300 electoral districts and a further 200 via proportional representation spread across five electoral regions. They will also choose six governors and several hundred state and municipal officials.

Polls will open on Sunday at 8:00 a.m. (1300 GMT) and close at 6 p.m. local time (2300 GMT).

Source: Xinhua