Labor unions play a central role in the Democratic party coalition, providing candidates with voters, volunteers, and contributions, as well as lobbying policymakers. Has the sustained decline of organized labor hurt Democrats in elections and shifted 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 from 1980 to 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 US Senate, US House, and Gubernatorial races, as well as on state legislative control. Turnout is also 2 to 3 percentage points lower in right-to-work counties after those laws pass. We next explore the mechanisms behind these effects, finding that right-to-work laws dampen organized labor campaign contributions to Democrats and that potential Democratic voters are less likely to be contacted to vote in right-to-work states. The weakening of unions also has large downstream effects both on who runs for office and on state legislative policy. Fewer working class candidates serve in state legislatures and Congress, and state policy moves in a more conservative direction following the passage of right-to-work laws.