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)