Mexico's opposition Institutional Revolution Party (PRI) is still keeping an opinion poll lead before looming mid-term legislative elections, but its lead is narrowing, according to a survey published on Wednesday.

The PRI, which ruled the country for 71 years until 2000, obtained 37 percent of preferences in a survey conducted by local newspaper Reforma, but its lead over the ruling National Action Party (PAN) has fallen to four percentage points from seven percentage points a month earlier.

The PAN had lost a percentage point of support but still remained in second place with 33 percent, according to the survey.

The survey, conducted among 1,515 registered voters across the nation from June 12 to 14, also showed that the Revolutionary Democratic Party (PRD) obtained 16 percent of preferences, the lowest among the three largest parties in the country.

The combined vote for all the other smaller parties was 14 percent, up from 12 percent in May.

The survey also showed rising support for the so-called "null vote" campaign, in which leading political figures called on citizens to spoil their ballots in protest against the parties that they say no longer represent citizens.

Some 15 percent of those surveyed said they had considered a null vote, compared with 10 percent a month earlier, and five percent spoiled the survey ballot paper, compared with 2.5 percent a month earlier.

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.

Mexicans will vote on July 5 to elect 300 deputies in 300 electoral districts.

Source: Xinhua