AGUASCALIENTES, Mexico — If there is a case to be made for preserving Mexico’s status quo, the citizens of Aguascalientes would seem to be the ones to make it.

A new Nissan plant is hiring for the night shift, and trains loaded with mechanical parts clatter north to the Texas border and beyond. Factory jobs abound, the crime rate is low, and even in the long-neglected eastern heights, a glass-walled public swimming pool crowns a sloping ribbon of parkland.

But as the presidential election nears, the discontent driving voters in the rest of the country has spread even to Mexico’s central manufacturing belt, despite its buoyant economy. The national rage over corruption that is imperiling the governing Institutional Revolutionary Party has taken hold in Aguascalientes, and citizens who could once be counted on to vote conservatively now appear ready to flip.

That helps explain the changing political fortunes of Andrés Manuel López Obrador, the populist presidential candidate. In his last two runs presidential runs, Mr. López Obrador barely registered here. This time, newfound support from voters in regions like Aguascalientes appears to be adding to his lead days before the July 1 vote.