We show that the effect of tax advantages of municipal bonds on the market microstructure of municipal bond auctions is a crucial determinant of state and local governments' borrowing costs. Reduced-form estimates show that increasing the tax advantage by 3 pp. lowers mean borrowing costs by 9-10%, consistent with a greater than-unity passthrough elasticity. Non-parametric evidence shows that strategic participation and bidding in imperfectly-competitive auctions generates this greater-than-unity passthrough. Using a structural auction model to evaluate the efficiency of Obama and Trump administration proposals, we find that the reduction in municipal borrowing costs is 2.8-times the revenue cost of the tax advantage.
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