MEXICO CITY (Reuters) - Time and again, Andres Manuel Lopez Obrador has proven that Mexico's left-wing parties cannot compete without him. But after his defeat in Sunday's presidential election, they may not be able to prosper with him either.
The most successful Mexican leftist in a generation has taken his followers to within sight of power in the last two presidential elections - only to fall short, reject the results and end up in a defiant stand-off with rivals.
Six years after he brought central areas of Mexico City to a standstill protesting the outcome of the 2006 election, Lopez Obrador is now challenging last Sunday's election result, raising fears of new chaos in the capital and deep divisions in Congress.
He has built up a loyal following that extends from the poorest sections of society to intellectuals who applaud his talk of ending corruption and eradicating poverty.
But he has fallen short of the votes needed to win power and seems unable to broaden his appeal much further, with many voters worried he would be a radical and divisive president.
His uncompromising attitude has sown division in the Left's ranks in the past, and could now restrict its ability to influence legislation as it prepares to enter the lower house of Congress as the second strongest force.
Lopez Obrador has accused winning candidate Enrique Pena Nieto and his opposition Institutional Revolutionary Party, or PRI, of conspiring with powerful business interests to steal the presidential election with vote buying and illegal funding.
As U.S. President Barack Obama and other world leaders congratulated Pena Nieto on his win, Lopez Obrador called him "totally immoral" and said everyone who voted for his rival, apart from the poor, were deliberately backing a "regime of corruption."
The words are unlikely to encourage cooperation between the two camps in Congress, where results suggest the centrist PRI has fallen short of an absolute majority, making it reliant on the support of other groups to pass bills.
Some senior leftist politicians are worried that they will again be sidelined.
"We shouldn't make the mistake we made in 2006," said Jesus Ortega, a former chairman of the Party of the Democratic Revolution, or PRD, the main leftist group in Congress and in the coalition that backed Lopez Obrador's candidacy.
"We can't isolate ourselves or get marginalized or hide in systemic opposition. We need to be actors in the deals and the negotiations for the progressive reforms the country needs."
The PRD, which also tightened its hold on Mexico City by winning more than 60 percent of the vote in an election for mayor on Sunday, is eager to use its strength to press the case for democratic, political and judicial reform in Mexico.
Boosted by a recent upsurge in student-led opposition to the PRI and its ties with Mexico's dominant media, the PRD is also riding a wave of popular demands to loosen the television stranglehold of broadcasters Televisa and TV Azteca.
Progress on these reforms could help curb the kind of abuses Lopez Obrador has linked to the PRI, which ruled Mexico for 71 years between 1929 and 2000, often resorting to vote-rigging, corruption and the repression of dissent to get its way.
But if Lopez Obrador pours more scorn on Pena Nieto and refuses to recognize him as president, Congress is likely to be dominated by Pena Nieto's efforts to strike deals on economic reforms with the biggest losers of Sunday's elections, President Felipe Calderon's conservative National Action Party, or PAN.
Famed as a defender of the poor and the downtrodden, Lopez Obrador has been a canny political operator when in power and forged a successful relationship with Carlos Slim, the world's richest man, while mayor of the capital.
Since cutting his teeth in politics in his home state of Tabasco in the 1970s and 1980s, the 58-year-old Lopez Obrador has noisily contested several votes, and political analysts are highly skeptical he will back down in 2012.
"This is the only thing he knows how to do," said Ulises Beltran of polling firm BGC. "Lopez Obrador is a loose cannon. The Left will again have real trouble uniting."
Lopez Obrador won 2.5 million votes more in 2006 than the leftist candidate received in the previous two presidential elections combined, and his combined vote tally in the past two elections was greater than that of the PAN or the PRI.
However, though half of Mexico's population is poor, he was unable to win over enough centrist voters and instead came in a close second in both races.
The PRD has been unable to win power since it was formed in the late 1980s by a mix of communists, socialists and dissident PRI politicians such as Lopez Obrador. Within the party, there are still at least six identifiable factions and the party must also contend with the push and pull of smaller leftist parties.
Supporters say that only Lopez Obrador can bring all of the disparate elements under one roof.
"He may not be the best leader, but he's the only one," said Jonathan Dominguez, an 18-year-old supporting Lopez Obrador's defiance of the voting authorities.
The power base of the Left is Mexico City, which the PRD has held since mayoral elections were first held there in 1997.
Lopez Obrador's popular administration cemented that hold on the metropolis but has also created competition for him there.
Current mayor Marcelo Ebrard, a moderate, challenged Lopez Obrador for the PRD's presidential nomination and narrowly lost, but is still touted as a potential president of Mexico - as is the man who will succeed him after a record victory in the mayoral election on Sunday, Miguel Angel Mancera.
Neither is a polarizing figure in the mold of Lopez Obrador and would likely appeal to more middle-class voters, but neither has the popular base that he has built up outside Mexico City.
Austere and priding himself on his incorruptibility, Lopez Obrador has never accepted his 2006 defeat by President Felipe Calderon of the ruling National Action Party (PAN).
After election authorities rejected his claims of fraud in 2006, Lopez Obrador declared "to hell with your institutions," and drew huge crowds of supporters onto the street. But public opinion turned against the protests.
In 2009, the Left's share of the vote fell from nearly a third to less than a fifth in congressional elections. But it did not make the bloc any more pliant in Congress.
So obstructive was Lopez Obrador to Calderon's economic reforms that it made him an important factor in the PRI's election victory on Sunday, said George W. Grayson, a Mexico expert at the College of William & Mary in Virginia.
"He attacks anyone who doesn't march to his drumbeat," said Grayson, a biographer of Lopez Obrador. "He thinks only he has the correct vision. This makes it very difficult for the Left."
Leftist victories last week have created new power bases with first-time captures of the states of Morelos and Tabasco.
Graco Ramirez, the winner in Morelos, wasted little time in advising Lopez Obrador not to stage another protracted protest. "I don't think it's what we need or what Andres Manuel needs."
Additional reporting by Michael O'Boyle and Gabriel Stargardter; Editing by Kieran Murray and Vicki Allen