By Dave Graham

CHIAPA DE CORZO, Mexico (Reuters) - For most of the past five years, Andres Manuel Lopez Obrador has journeyed far and wide across Mexico, railing against the corruption, fraud and injustice he says cost him the presidency.

Seething over his wafer-thin election loss to conservative Felipe Calderon in 2006, the fiery leftist shook Mexico with some of the biggest street protests in its history, damning its institutions and declaring himself the rightful president.

Yet since recently winning the support of Mexico's main leftist parties to run again for president, Lopez Obrador has made a sharp U-turn, preaching love and forgiveness to win back voters who had been turned off by his protests and clamor.

The former mayor of Mexico City flanked his new approach with a drive to reach out to private business and reassure voters he will not put the economy at risk, charges that cost him dear in the final stages of the last election campaign, when his lead over Calderon evaporated.

The new message has forced Lopez Obrador into a delicate balancing act, struggling to convince waverers to follow him down the road to the "loving republic" he now invokes, while still firing up his core supporters.

On campaign last week in the southern state of Chiapas, Lopez Obrador set out his stall as a man of peace, urging his followers to "practice love of thy neighbor" and insisting his rivals were not enemies but people "just as desperate" as the rest in Mexico, where around half the population is poor.

Then, in an instant, he switched into tearing up his adversaries, conjuring up visions of decadence, slavery and dictatorship if voters fail to heed his calls for change.

Particular scorn was reserved for the opposition Institutional Revolutionary Party (PRI), which leads the race, drawing on much of the same base of support as Lopez Obrador.

"If the PRI comes back it would be terrible," Lopez Obrador shouted in the town of Chiapa de Corzo. "It would be like the return of that great dictator, Antonio Lopez de Santa Anna."

Best known in the United States for his attack on the Alamo in Texas in 1836, general Santa Anna has gone down in Mexican history as a dictator blamed for the loss of half the country's territory to the giant northern neighbor in the 1830s and 1840s.

BEST KNOWN POLITICIAN

Lopez Obrador led the field for most of the 2006 campaign but most opinion polls this time around show him with support of just under 20 percent.

"He's pursuing a really difficult strategy, because it's a very abrupt change to go from a hard, combative politician to a conciliatory one," said Federico Berrueto, director general of polling firm Gabinete de Comunicacion Estrategica (GCE).

Fronting a leftist alliance dominated by the Party of the Democratic Revolution (PRD), Lopez Obrador has pledged to create 7 million jobs to keep young people out of the clutches of drug gangs, whose turf wars during a crackdown launched by Calderon's government have claimed 47,000 lives, damaging Mexico's reputation with tourists and investors.

Lopez Obrador is pledging to revitalize state oil monopoly Pemex, guarantee university education for all, slash the pay of senior government officials and oversee economic growth of six percent per year.

He has a very loyal core of supporters but many Mexicans wonder whether he can build enough bridges with business and other political parties after his cries of foul play brought swathes of the capital to a halt in 2006.

"People got tired of them," said Adela Mendoza, a 53-year-old from Nezahualcoyotl, a gritty district on Mexico City's western flank. Mendoza voted for Lopez Obrador in 2006, but says she has given up him now.

Lopez Obrador has not given up on Mexicans.

Continuing to remind them of the "great fraud" on his tour of Chiapas, he raced hundreds of miles (km) a day on bumpy roads, dirt tracks and mountain passes to reach remote towns and villages other politicians rarely, if ever, venture into.

His advisers say he has visited all of Mexico's 2,440 municipalities an average of two times since the 2006 vote, making him the best known politician in the contest.

Some polls show the 58-year-old father of four to be more widely recognized than the president himself.

To his supporters, the austere Lopez Obrador is the only man with the moral authority to end the corruption and inequality that has long blighted Mexico.

"He has a simple way of expressing himself, he's sincere and he talks to you honestly," said Javier Villareal Cruz, 40, under the roof of a sports hall in the town of Cintalapa.

Cheers broke out across the basketball courts when Lopez Obrador told a crowd of several hundred flag-waving supporters he would end the "decadence" of overpaid public servants.

"They have plastic surgery on the backs of the people," Lopez Obrador shouted. "They have helicopters, planes. They give themselves a great life."

Zeferino Morales, 62, a farmer from the Zoque ethnic minority, traveled six hours to see Lopez Obrador in nearby Copainala pledging to establish state pensions for the elderly across Mexico and wean the country off foreign food imports.

"The presidents have abandoned us here," he said in halting Spanish. "Lopez Obrador is going to change a lot of things."

HIGH COST

Yet to many Mexicans, Lopez Obrador is the one politician for whom they will not vote.

Tainted by depictions in the media as a belligerent holding Mexico to ransom, negative ratings for Lopez Obrador among voters outweigh positive ones by a ratio of three to two, figures from pollster Mitofsky showed this month.

"If he'd taken a break, he might have started from a very low base, but he wouldn't have had such a negative image," said Berrueto at GCE. "But he stayed so active and present."

By contrast, presidential front-runner Enrique Pena Nieto of the PRI has a ratio of more than four to one in his favor.

The telegenic Pena Nieto, a 45-year-old former governor of the State of Mexico, has helped to rejuvenate the PRI, a party that ruled the country for 71 straight years up to 2000, by which time it had become a byword for corruption.

Lopez Obrador faces a stronger rival in Pena Nieto than the PRI fielded in 2006, when the party suffered its worst ever result with the candidacy of Roberto Madrazo.

The front-runner for the candidacy of Calderon's conservative National Action Party (PAN), Josefina Vazquez Mota, offers voters another break with the past as a female contender.

Lopez Obrador insists he was right to take to the streets of Mexico City in 2006, arguing it prevented violence erupting.

But in an interview with Reuters in Tapachula, a city far from the capital on the Guatemalan border, he admitted that the media impact of his protests had cost him "a lot."

"There are people in Tapachula who still question me about it," he said. "Even though they weren't there."

(Editing by Kieran Murray)