Hundreds of people in 15 cities across the country rallied on June 19 to call for a “Robin Hood tax,” a small tax on financial transactions that activists and economists say could raise billions of dollars in much needed revenue and discourage risky trades on Wall Street. The rallies were the kick-off to a major campaign led by the National Nurses Union to institute the tax.

The union, the nation’s largest for nurses, says that a Robin Hood tax on speculative Wall Street transactions in stocks, bonds and currencies could raise money for health care, housing, jobs and education. The union and its allies are proposing a tax of around a half of 1 percent to be levied on Wall Street trades. Dozens of other organizations have signed onto the campaign, from the climate change group 350.org to Public Citizen to the Progressive Democrats of America. The campaign is global, with activists in countries from France to Canada pushing for a similar financial transaction tax.