Andrés Manuel López Obrador rails against "the privileged" in a staccato voice which softens as he turns to the virtues of "the poor" and a cheeky grin accompanies the thumbs up to go with his slogan: "Smile, we are going to win."

Depending on who you talk to, the presidential candidate, who holds a narrow lead in the opinion polls ahead of Sunday's Mexican elections, is the great hope of the downtrodden, a messianic danger to stability or a crafty pragmatist. This son of a shopkeeper from the marshy backwaters of south-eastern Mexico is striking a chord and inciting discord in a country where half the population is poor and the richest 10% own 45% of the wealth.

Mexico City Mayor Andres Manuel Lopez Obrador gives a thumbs up to supporters in Mexico City. (AP Photo)

As the election approaches, opinion polls give the candidate of the Party of Democratic Revolution a slight lead over Felipe Calderón of the governing centre-right National Action party.

If Mr López Obrador wins, Latin America's much talked about leftward march - through Venezuela, Brazil, Argentina, Uruguay, Bolivia and Chile - will have reached the United States' back door. If he loses, in the wake of electoral disappointments in Colombia and Peru, it will look like the retreat has begun.

Not that there is much uniformity among the region's leftwing leaders from the pugnacious bravado of Hugo Chávez in Venezuela to the eminently reasonable Michelle Bachelet in Chile. Most observers put Mr López Obrador somewhere in the middle.

The 52-year-old former mayor of Mexico City is a strong leader with a certain magnetism, a sense of mission and an apparently limitless belief in himself. But he also seems happy to accept that at least half of Mexico's electorate is traditionally conservative, which, together with the country's 2,000-mile border with the US, puts important limits on how far left he can go without a backlash.

So though his rhetoric can sound radical, Mr López Obrador's policy proposals are hardly revolutionary. They include food handouts for the elderly, household energy subsidies and massive investment in infrastructure to kick-start Mexico's sluggish economy - all to be paid for by cutting top bureaucratic salaries in half, including the president's.

"He will head a progressive government," says Manuel Camacho, one of the candidate's small inner circle. "It is not possible for a genuinely leftwing government to take power in Mexico, not via the ballot box, and not through force either."

The Bush administration seems to agree. Though it openly voices concern about President Chávez and his Bolivian ally Evo Morales, there is no sign of alarm at the possibility of a López Obrador victory. Not so among Mexico's business community, which spits fire at his name and has thrown its weight behind his main rival, Mr Calderón.

A comparatively grey candidate with little obvious political charm, Mr Calderón, 43, promises job creation through market-led reforms in a stable environment. But polls show that his chance of winning rests on the success of a negative campaign to convince voters that Mr López Obrador is a closet authoritarian prepared to throw fiscal responsibility to the wind and plunge the country into economic crisis.

Few Mexicans can forget the devastating crisis of 1995, the last of the meltdowns that dogged the final decades of the deeply corrupt single-party regime that installed itself in power in 1929. The Institutional Revolutionary party (PRI) kept political control for 71 years through patronage, selective repression and electoral fraud. Its legendary electoral machine has kept its candidate, Roberto Madrazo, only a few points behind the frontrunners today.

The PRI lost the last presidential elections in 2000 to Vicente Fox of the PAN, who unified opposition to the regime behind his slogan "Change".

President Fox leaves office appreciated by the public for keeping crisis at bay, being pleasant and relatively honest. But he also departs vilified as weak and ineffective, having failed to push through most of his reform programme.

Mr Fox's limp legacy allows Mr López Obrador to claim he is the "real change" that the electorate has wanted all along. With many of his diehard supporters convinced that only fraud can snatch victory from their hero, the possibility of tension on the streets is real if his main rival wins by a narrow margin.

Love him, hate him or just watch him, Mr López Obrador is the candidate to beat, and may well remain at the centre of the post-electoral show even if he loses.

Mexico

Population 106.4 million, of which about 71.4 million are eligible to vote

Main language Spanish

Main religion Christianity

Life expectancy 72 (men), 77 (women)

Ethnicity Mestizo (Amerindian-Spanish) 60%, mainly Amerindian 30%, Caucasian 9%, other 1%

Main exports Machinery and transport equipment, fuels and lubricants, food and live animals. Nearly a third of government revenue comes from oil exports, mostly to the US

Per capita income US $6,790, the highest in Latin America.

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