The IRS is investigating whether five wealthy donors financing political advocacy groups should be paying additional taxes on their contributions. What is this tax, and why hasn't this been enforced before?

The Internal Revenue Service is currently investigating whether the contributions of five wealthy donors financing political advocacy groups are subject to a gift tax. But what is a gift tax and why hasn’t this been enforced before? Are the wealthiest contributors skimping on taxes they owe? On this week’s edition of The Breakdown, tax law expert Donald Tobin joins DC editor Chris Hayes to discuss the different subsets of taxable entities under campaign finance law and how the IRS investigation may impact the upcoming election cycle.

Further Reading: Donald Tobin’s paper on possible tax loopholes in campaign finance. New York Times article on the IRS investigation of political advocacy groups.

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