Scholarship on bureaucratic responsiveness to Congress typically focuses on delegation and formal oversight hearings. Overlooked are daily requests to executive agencies made by legislators that propose policies, communicate concerns, and request information or services. Analyzing over 24,000 of these requests made to 13 executive agencies between 2007 and 2014, I find agencies systematically prioritize the policy-related requests of majority party legislators-but that this effect can be counteracted when presidents politicize agencies through appointments. An increase in politicization produces a favorable agency bias toward presidential copartisans. This same politicization, however, has a net negative impact on agency responsiveness-agencies are less responsive to members of Congress, but even less responsive to legislators who are not presidential copartisans. Critically, this negative impact extends beyond policy-related requests to cases of constituency service. The results suggest that presidential appointees play an important, daily mediating role between Congress and the bureaucracy.