This Congress is the most diverse ever. But Hill staffers remain overwhelmingly white.

Last month, when Michael J. Brewer became one of 10 communications directors of color out of roughly 100 in the U.S. Senate, he took it as a small sign of progress.

Brewer, who runs communications for Democrat Raphael Warnock, Georgia’s first Black senator, knows most of his work for the freshman lawmaker will be done behind the curtain. Still, he’s acutely aware of the importance of his presence: He is currently one of just two Black communications directors in the Senate — and the only Black man to hold the...