KEY POINTS
  • Sen. Ron Wyden's proposal would have people pay yearly taxes on assets that gain in value.
  • The tax would apply to assets traded in liquid markets, like stocks and bonds, and to illiquid assets like real estate, private companies and complex investments.
  • A tax on unrealized gains would be not only difficult to implement but also could devastate markets.

Republican Sen. Pat Toomey of Pennsylvania has called it a "breathtakingly terrible idea," and he is absolutely, 100 percent right.

Sen. Ron Wyden, D-Ore., has proposed a so-called mark-to-market version of the capital gains tax. Put more simply, investors would pay capital gains taxes each and every year in which their assets go up in value, instead of only when they are sold.