Polls taken in mid-June show Andres Manuel Lopez Obrador, Mexico City's leftist former mayor, only two points ahead of conservative, ruling party candidate Felipe Calderon.
The 36-34 percent near-tie has made the run-up to the July 2 vote one of the closest in Mexico's history and is keeping pollsters in the United States busy considering the degree of influence the winner could have on increasingly sensitive U.S.-Mexico ties.
"At stake is the future course of America's influential southern neighbor at a time when Mexico and the U.S. wrangle over border security and immigration issues, and as other Latin American nations turn to left-leaning leaders," U.S. opinion pollster John Zogby told the Reuters news service.
In the last six years, aside from splitting with the Bush administration on the war in Iraq, outgoing President Vicente Fox has been a key U.S. ally. President Bush and Fox have consulted on border and immigration issues and have come together as economic partners in the North American Free Trade Agreement.
A victory by Calderon could continue that trend. Though the former energy minister and pro-business leader from the wealthy north has criticized U.S. plans to build a nearly 2000-mile long fence along its border with Mexico, he also has used his close relationship with Fox to enhance his image, insisting he would follow in Fox's foreign policy and fiscal footsteps if elected.
"Obviously, the United States would be more comfortable with Calderon because he is going to be much more predictable and he also is more in tune with the U.S. model for the Mexican economy," Pamela Starr of the Eurasia consulting group told Reuters.
"But, at the same time, the United States is more than willing to work with Lopez Obrador," Starr said.
Lopez Obrador supporters -- ignoring comparisons of their candidate to Venezuelan President Hugo Chavez, who has sided against the United States with Cuban leader Fidel Castro and newly elected Bolivian President Evo Morales -- say the feeling is mutual.
"He's not going to be gratuitously aggressive. He's not going to be openly anti-American, no president of Mexico at this stage, with NAFTA, could afford to be," former ambassador Andres Rozental told Reuters.
But Lopez Obrador has rankled U.S. leaders by promising to oppose a NAFTA agreement dropping tariffs on U.S. corn and bean imports to Mexico and has said he will not be a "puppet" for the United States.
Still, he has tried to distance himself from the anti-American policies of his Latin American counterparts.
"We have 20 million Mexicans in the U.S. Eighty percent of our international trade is with the U.S. So [our policy] has to be different," Lopez Obrador told The Washington Post.
So far, Lopez Obrador's heaviest support is coming from the south, home to a majority of Mexico's poor. Calderon's base is in the north, where his National Action Party is gaining popularity.
With such a slight edge, the key for Lopez Obrador will be to further the support he has gained on the campaign trail by mobilizing his party base on Election Day. The same goes for Calderon.
If not, a third candidate, Roberto Madrazo, candidate for the Institutional Revolutionary Party, or PRI, which ran Mexico unchallenged for 71 years, just may surprise analysts and run away with the vote.