After 99 percent of the vote has been tallied in the preliminary count in Mexico's presidential election, the narrower-than-expected margin of victory for Enrique Peña Nieto is fueling suspicion among Andres Manuel López Obrador's followers about the fairness of the vote, and he refused Monday night to concede defeat.

Lopez Obrador's defiance is similar to when he lost a razor-thin race in the 2006 presidential race and set off months of political unrest. Although this time, he has not called his followers into the streets to protest.

López Obrador of the Democratic Revolution Party trails Peña Nieto of the Institutional Revolutionary Party, or PRI by six percentage points.

The leftist candidate was supposed to lose by a double digit margin to the apparent victor Peña Nieto.

López Obrador argued from the start of the campaign that pollsters were manipulating pre-election surveys to favor Peña Nieto as a way to boost the idea that the PRI candidate was far out in front.

The media sponsored Peña Nieto, they manipulated, they deceived. This was a really dirty election.

- Andres Manuel López Obrador, Candidate

Pollsters denied that, and said Monday that they suspected some voters changed their minds and switched to López Obrador in the final week before Sunday's election. Mexican electoral law bans the publication of polls just before elections, something the polling firms said prevented them from getting a last-minute snapshot of voter sentiment.

The leftist candidate also complained throughout the campaign that biased media favored Peña Nieto, particularly Mexico's semi-monopolized television industry.

"The media sponsored Peña Nieto, they manipulated, they deceived," López Obrador said at a news conference Monday evening. "This was a really dirty election."

López Obrador said he would not accept the preliminary election results reported by the Federal Elections Institute and would wait until Wednesday, when the official results are to be announced, before deciding what he will do.

"We will not accept a fraudulent result," he said.

López Obrador said he probably would challenge Sunday's vote results, but didn't say if he would try to repeat the nearly two months of street blockades in Mexico City that he led in 2006 to protest his close loss to President Felipe Calderón of the conservative National Action Party that the leftist also attributed to fraud.

Pre-election polls said Peña Nieto was favored by 32 percent to 41 percent of voters, while López Obrador had support ranging from nearly 24 percent to 25 percent. Josefina Vazquez Mota of the National Action Party was third at around 19 percent or 21 percent.

IFE's preliminary vote count, however, has produced a much tighter contest. Peña Nieto leads with 38 percent of the votes, while López Obrador is much closer than predicted with 32 percent and Vazquez Mota has 25 percent.

"To say that more than 200 polls, including some that were done by some polling companies he trusted, were manipulated is absurd," said Roy Campos, president of the polling firm Mitofsky.

Campos said polls estimated correctly in what place each of the candidates would finish. The gap between López Obrador and Peña Nieto narrowed because at least one in nine voters changes his or her mind at the last minute, he said.

"There is definitely an effect toward the end that we are not able to measure because the last survey is done the weekend before the election," Campos said.

Jorge Buendia of the polling firm Buendia and Laredo also said it appeared that some voters who initially supported Peña Nieto changed their minds.

"Sometimes we forget that people often tell you a preference without being completely convinced," Buendia said. "But it's not up to us as pollsters, or democrats, to decide that a convinced vote is superior to a doubtful vote."

Supporters of López Obrador, who narrowly lost the 2006 election by a half percentage point, say the polls were "propaganda" used against him and many are urging him to declare the election a fraud.

"We have a situation where the numbers were very different from what the propaganda of the polls was spreading for three months," Manuel Camacho Solis, a former Mexico City mayor who was among López Obrador's campaign coordinators in 2006 and now leads an informal coalition of leftist parties, told reporters Monday.

"They first prescribed us a difference of 25 points, then of 20, and toward the last days of the campaign, and almost in a generous way, they were talking of 15 percentage points."

Hundreds of young people gathered Monday at a monument along Mexico City's main Reforma Avenue to protest Peña Nieto's victory, which they called the result of electoral fraud.

"The election results are being manipulated by the media," said Vladimir Cervantes, a 23-year-old university student. "We will resist so that they don't make the fraud official."

The students said they knew of at least 500 reports of irregularities that were captured in photos and video, including the buying of votes.

"What we want is for the truth to come up and to stop Peña Nieto from taking the presidency," Cervantes said.

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