What is the relationship between economic and political inequality? Campaign contributions are often mentioned among the possible channels creating opportunities for richer people to exert disproportionate influence on policymakers. At the same time, by exacerbating economic disparities, public policies that favour the wealthy might also give them a greater relative weight in the donor pool, hence creating a self-reinforcing spiral between material wealth and political influence. Using data from the US at the census tract level, we present two types of evidence on the link between economic inequality and the concentration of campaign contributions. First, we document a number of socio-economic patterns in the evolution over time of campaign contributions during the period 1985-2014, showing that growing income inequality is associated with an increased concentration of donations at the top of the income distribution. Then, we analyze the effect of the 1986 Tax Reform Act, a remarkable tax cut that decreased the marginal tax rates disproportionately at the top of the income distribution. We show that this policy decision caused a spike in contributions from citizens in the top 10 percent of the income distribution, thus rising their political clout. Our conclusion is that the Tax Reform Act, a landmark policy of the second Reagan administration, has been a crucial step in the spiral between economic inequality and uneven political influence of the last four decades.