Bernie Sanders wants to raise a lot of money from Wall Street.

Mr. Sanders’s plan isn’t to ask Wall Street for money. It’s to demand it in the form of a financial-transaction tax.

Proponents claim the tax would collect tens of billions of dollars, discourage speculative and high-frequency trading, and make markets safer and less volatile. Its opponents say the revenue estimates are overstated and the tax will actually make markets more volatile.

Mr. Sanders wants to use funds raised by this tax to pay for his college-education agenda, which includes making public colleges free, cutting interest rates on student loans and increasing financial aid. The costs of these proposals—$75 billion per year by Mr. Sanders’s count—could be fully paid by the tax, according to Mr. Sanders.

Under a bill Mr. Sanders introduced last May, investors would be required to pay an excise tax on any transfer of a stock, bond, partnership interest or derivative. Stock trades would incur a 0.5% tax rate, or $5 for every $1000 of stocks traded. Bond trading gets a 0.10% tax rate. Derivatives a 0.005% tax rate.