By Miguel Angel Gutierrez and Lizbeth Diaz
MEXICO CITY (Reuters) - With just a week to go until Mexico's presidential election, leftist contender Andres Manuel Lopez Obrador has stirred up fears of a repeat of the chaos caused when he led massive protests after his narrow defeat in 2006.
As his hopes of victory fade, the former mayor of Mexico City has talked up the prospect of voter fraud robbing him of victory on July 1 - the same accusation he made six years ago when he brought much of the capital to a standstill for weeks.
Fueled by a burst of student-led opposition to front-runner Enrique Pena Nieto of the opposition Institutional Revolutionary Party (PRI), Lopez Obrador saw a poll rally in late May, with one showing him just four percentage points adrift of his rival.
That surge has now ended and most polls show him to be at least 10 points behind Pena Nieto. Some recent surveys even show Lopez Obrador to have fallen back behind Josefina Vazquez Mota of the ruling National Action Party (PAN).
However, the 58-year-old says his campaign's data show him to be slightly ahead, and he has told supporters to look out for a repeat of the tricks he says denied him victory in 2006.
Another close finish would raise the risk of a new post-vote crisis and likely trigger legal challenges and street protests. But if Pena Nieto wins by the wide margin many polls are predicting, any fraud claims would ring hollow and unrest would likely be short-lived and on a smaller scale.
Lopez Obrador and his campaign team last week accused the PRI of planning to pay voters to cast ballots for Pena Nieto and asserted that state governors from the centrist PRI had agreed to guarantee their candidate a certain number of votes.
"Buying votes is a crime. We are going to issue a call to avoid election fraud," Lopez Obrador told supporters at a rally in the central state of Guanajuato. His campaign also raised the prospect of fraud by Vazquez Mota and the PAN.
His comments stoked a war of words with the PRI, which dismissed the claims and accused him of preparing another post-election conflict without offering any proof of wrongdoing.
Often dubbed "Peje", a tough type of swamp fish in his native state of Tabasco, Lopez Obrador almost won in 2006 by leading mass marches and condemning Mexico's "mafia" of rich and powerful oligarchs.
But he alienated many supporters after losing to Felipe Calderon of the PAN by just 0.56 percentage points. Lopez Obrador declared himself the "legitimate president" and his protests shut down central parts of Mexico City for weeks.
He has toned down his rhetoric in this campaign, calling for a "loving republic" and reaching out to business groups. But he has also sent contradictory messages on whether he will launch protests if he loses, saying he will respect the results while also accusing his rivals of preparing election fraud.
Lopez Obrador's aides say they will not remain idle if there is evidence of fraud.
"The last thing we want to do is what we did six years ago, and I'm sure it won't happen. But it will also depend on what (our rivals) do," said his close aide Manuel Camacho. "If there are irregularities during the election, of course we will go to court and there will be dissent."
The May surge in support for Lopez Obrador drew on a flurry of youth activism against Pena Nieto and the PRI, whose 71-year rule was tainted by cronyism, corruption and authoritarianism. The PAN ended the PRI's hold on power in an election in 2000.
The most prominent student group 'Yo soy 132' (I am 132), helped to bring out tens of thousands of protesters against Pena Nieto during the campaign, but has said it won't mobilize more demonstrations unless it suspects fraud.
Election authorities insist there is no room for vote rigging, particularly given that members of the main parties will be monitoring more than 143,000 voting booths.
Mexican financial markets have bet on a Pena Nieto win, so a close finish that puts his mandate and economic reforms at risk could fray nerves and hit asset prices.
Pedro Tuesta, an economist with 4Cast in Washington, said he does not expect a fresh post-election crisis as long as the winner has a margin of at least 5 percentage points.
With the chaos of Lopez Obrador's protests that clogged up the capital's main avenue Reforma still seared into the public consciousness, many Mexicans are holding their breath.
"I hope the man keeps his word. It would really affect us if they close down (Reforma) again," said Carlos Equino, 31, who sells snacks in the city's sprawling Chapultepec Park. "Let's hope he's a gentleman this time and just like he told people to take to the streets, this time tells them not to."
(With reporting by Tomas Sarmiento, Noe Torres and Lorena Segura; Editing by Simon Gardner, Kieran Murray and David Brunnstrom)