We present a formal model explaining that US presidents strategically unionize federal employees to reduce bureaucratic turnover and ‘anchor’ the ideological composition of like-minded agency workforces. To test our model’s predictions, we advance a method of estimating bureaucratic ideology via the campaign contributions of federal employees; we then use these bureaucratic ideal point estimates in a comprehensive empirical test of our model. Consistent with our model’s predictions, our empirical tests find that federal employee unionization stifles agency turnover, suppresses ideological volatility when the president’s partisanship changes, and occurs more frequently in agencies ideologically proximate to the president.