Labor unions directly affect wages, employment, industrial structure, and inequality. But unions also influence the economy and labor market indirectly through their effects on politics, providing candidates with voters, volunteers, and contributions, and lobbying on public policy. We use the enactment of right-to-work laws---which weaken unions by removing agency shop protections---to estimate the effect of unions on politics and policy from 1980-2016. Comparing counties on either side of a state and right-to-work border to causally identify the effects of the state laws, we find that right-to-work laws reduce Democratic Presidential vote shares by 3.5 percentage points. We find similar effects in Senate, House, and Gubernatorial races, as well as on state legislative control. Turnout is also 2 percentage points lower in right-to-work counties after passage. Exploring the mechanisms, we find that right-to-work laws dampen organized labor contributions to Democrats and that potential Democratic voters are less likely to be contacted to vote. The weakening of unions also has large downstream effects: fewer working-class candidates serve in state legislatures and Congress, while state policy moves in a more conservative direction.