RICHMOND, Calif. — This small, blue-collar city best known for its Chevron refinery has become the unlikely vanguard for anticorporate, left-wing activism in recent years, having seized the mantle from places like Berkeley, just south of here, or San Francisco, across the Bay.

It became the largest American city to be led by a Green Party mayor, who was re-elected two years ago even though the oil giant bankrolled rival efforts with $1.2 million. Social activists belonging to the Richmond Progressive Alliance gained control of the City Council, from where they have been taking on what they refer to as the “Chevron Man.”

But this election season, city leaders are fighting on two fronts, against not only Big Oil but also Big Soda, as they like to call their foes. If voters here approve a proposal on Tuesday’s ballot, Richmond will become the first city in the United States to add a tax on businesses that sell soda and other sweetened drinks, although many states already collect taxes on such drinks directly as part of anti-obesity efforts.

Fierce campaigning has brought in the kind of money rarely seen in a community of 104,000 people. Soda companies have funneled $2.5 million into efforts to defeat the tax, or Measure N, while supporters have raised only $69,000.