In the final stretch of Chicago’s closely watched mayoral race, candidate Paul Vallas is attacking his progressive opponent’s plan to fund public schools and infrastructure by taxing the wealthy — including a tax on financial trading that would hit some of Vallas’s top campaign donors.

The revenue plan proposed by Cook County commissioner Brandon Johnson includes what he calls a “Big Banks Securities and Speculation Tax,” which would levy a $1 or $2 charge on most trades. Johnson’s campaign estimates this financial transaction tax could raise as much as $100 million annually for the city.

Vallas opposed Johnson’s tax plan during a debate last week, arguing that raising taxes “is the absolute wrong approach to take,” and that Chicago’s next mayor should instead focus on reducing spending.