* Lopez Obrador says to run for Mexico's left in 2012
* Second run at presidency after narrowly losing 2006 poll
* Deal avoids splitting left-wing vote in election (Adds quote, details, background)
By Miguel Angel Gutierrez
MEXICO CITY, Nov 15 (Reuters) - Left-wing firebrand Andres Manuel Lopez Obrador will run again for the Mexican presidency in 2012, six years after his wafer-thin loss in the last election sparked massive street protests.
Lopez Obrador, 58, a champion of the poor in a country where nearly half the population lives below the poverty line, on Tuesday beat Mexico City Mayor Marcelo Ebrard in a survey carried out among 6,000 supporters of left-wing parties.
"To sum up friends, as free and responsible men and women we all aspire to live in a more just, a more humane and a more equal society," Lopez Obrador told a news conference.
Ebrard, his more moderate rival, said he would respect the results of the study and throw his support behind Lopez Obrador in order to avoid splitting the left-wing vote.
The survey was to see which of the two had more grass-roots support and both men said they would let the results decide. It asked five questions and Lopez Obrador, whose radicalism rattled markets in the last campaign, won three of them.
Painted by some opponents as a threat to private business, Lopez Obrador staged huge protests in Mexico City after the 2006 election, saying he had lost because of fraud.
His conservative opponent, President Felipe Calderon, rode out the storm and Lopez Obrador's popularity slumped after voters tired of the demonstrations which brought parts of the capital to a halt, including its main thoroughfare.
Having spooked investors during the 2006 presidential race with his fiery left-wing rhetoric, Lopez Obrador has sought this time to present himself as a moderate, courting business leaders and offering programs to boost small enterprises. <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Factbox on political risks in Mexico: [ID:nRISKMX]
Factbox on Lopez Obrador: [ID:nN1E7A91E8]
It remains doubtful whether he will have a similar impact on next year's result. Opinion polls show Lopez Obrador is trailing way behind frontrunner Enrique Pena Nieto of the main opposition Institutional Revolutionary Party (PRI).
"If he had not closed down (Avenue) Reforma years ago he would be a much stronger candidate today; he's still paying for that mistake," said office worker Ramon Lopez, 48.
A head-to-head survey in daily Excelsior on Oct. 31 showed Pena Nieto winning 55 percent of votes versus 26 percent for hopeful Josefina Vazquez Mota of Calderon's National Action Party (PAN), and just 19 percent for Lopez Obrador.
BOOST FOR PRI?
Analysts said the choice of Lopez Obrador over Ebrard, who made his name with progressive laws in the country's capital that legalized abortion and gay marriage, would likely favor Pena Nieto, who has yet to be formally chosen by the PRI.
"With Lopez Obrador as candidate, his supporters are celebrating and so is the PRI," said political scientist Jose Antonio Crespo from graduate school CIDE.
"We'll have to see what kind of different rhetoric Lopez Obrador might have compared to 2006. He has changed a bit, but I don't know whether it will be enough to get back the non-aligned voters, who were with him in 2006 but who he has lost. Now most of the non-aligned voters are with the PRI."
Analysts estimate non-aligned voters are 35 percent to 45 percent of the electorate. Lopez Obrador is seen as having little chance of winning the backing of PAN moderates.
The PRI, which governed Mexico for 71 years until 2000, can also hope to capitalize on Lopez Obrador's reputation.
As one of the best-known politicians in the country, Lopez Obrador has his work cut out to change people's minds about him. Due to his stormy past, many polls have shown he has the highest negative ratings of the leading contenders.
Mexico's main left-wing party, the Party of the Democratic Revolution (PRD), is a waning force in politics and the loss of two state governorships this year, including Michoacan on Sunday, has fanned internal unrest. [ID:nN1E7AD01E]
The completion of the survey, which was conducted by separate pollsters, suggests that the left will skip the process of internal pre-selections, which the electoral authority has scheduled in February for all parties.
(Additional reporting by Cyntia Barrera Diaz and Anahi Rama, Writing by Krista Hughes; Editing by Eric Walsh and Cynthia Osterman)