MEXICO CITY (Reuters) -- Mexico's ruling party presidential candidate Felipe Calderon has streaked into a lead of 10 percentage points over his main leftist rival in an opinion poll published on Thursday.

The latest survey was the fourth this week to put conservative Calderon ahead of his main rival, left-wing former Mexico City mayor Andres Manuel Lopez Obrador.

Calderon trailed for months but has surged after a strong performance in a televised presidential debate last week and aggressive media advertisements accusing his rival of being a populist who would ruin Mexico's finances.

Calderon, from the National Action Party, is a balding lawyer with a Harvard degree in public administration who is accused by critics of lacking charisma.

He promises to maintain the macroeconomic stability Mexico has won under President Vicente Fox and take a tough stand against crime.

Thursday's poll by the GEA-ISA firm put Calderon, a former energy minister under Fox, at 41 percent, up five percentage points from March.

Lopez Obrador, who has lost support after repeatedly sniping at Fox for campaigning on behalf of Calderon, was down three points with 31 percent.

Roberto Madrazo of the main opposition Institutional Revolutionary Party, which ruled Mexico for 71 years before Fox defeated it at elections in 2000, came in third at 25 percent.

Other polls this week show Calderon in the lead for the July 2 election, although the size of his advantage has ranged from 1 to 10 percentage points.

Thursday's poll was taken among 1,440 people just after the presidential debate, which Calderon was widely seen as having won. The survey had a margin of error of 3 points.

Mexico's law forbids Fox from standing for office again.