MEXICO CITY — The message that circulated on social media earlier this year — most Mexicans would have to re-register within days if they wanted to vote in the presidential election — set off a low-level panic on Facebook, Twitter and other platforms.

The thing is, it wasn’t true.

The source remains unclear. But whether the message was a dirty trickster’s attempt to undermine the system or just an ill-informed public service effort, the anger and uncertainty it generated represented an early skirmish in the battle over disinformation in this year’s fiercely contested election season.

“What candidates do on social media will be decisive,” said Carlos Merlo, managing partner of Victory Lab, a marketing firm dedicated to spreading viral claims, saying they must respond quicker than ever to combat disinformation.

There is a lot at stake in the July 1 vote: over 3,400 elected positions at the local, state and federal levels, more than in any other election in Mexican history. The biggest prize is the presidency, with five candidates on the ballot competing to succeed President Enrique Peña Nieto for a six-year term.