Mexican conservative presidential candidate Felipe Calderon had a four-point lead in an opinion poll published by the El Universal newspapertoday.
Mr Calderon, from President Vicente Fox's National Action Party, was at 39 per cent, up from 34 per cent in an El Universalpoll a month ago.
Andres Manuel Lopez Obrador, the leftist former mayor of Mexico City, had been in the lead but dropped three points to 35 per cent, the poll showed.
Mr Lopez Obrador had long been ahead in the race for the July 2nd election, but Mr Calderon has surged in all major polls in recent weeks after a strong performance in a televised presidential debate and aggressive campaign ads accusing his rival of being a populist who would ruin Mexico's economy.
The leftist's rivals have also linked him to a peasant riot in a town near the capital two weeks ago, a charge Mr Lopez Obrador strongly denies.
Roberto Madrazo of the Institutional Revolutionary Party came in third place in today's poll, down 4 points to 21 per cent.
The election campaign has turned nasty since Mr Calderon's team began attacking Mr Lopez Obrador, who promises to give priority to Mexico's millions of poor if elected.
He has accused Mr Fox of illegally backing Mr Calderon, his former energy minister. Mr Fox is legally barred from running for re-election.