A coalition of nurses’ unions is calling for a “Robin Hood†tax on Wall Street, which they say could generate up to $350 billion a year, in the first major protest ahead of this weekend’s NATO summit in Chicago. Their pitch: impose a tax of 50 cents on every $100 of trades of stocks, bonds, dividends and other financial transactions, which are not currently taxed. The U.S. would join more than a dozen other nations that already have a financial transaction tax.