The staff on Capitol Hill is too white. That’s what a group of frustrated members, lobbyists and aides are claiming as they press congressional leaders to adopt a version of the so-called Rooney rule. The rule, named after Pittsburgh Steelers owner Dan Rooney, has been credited with significantly increasing the number of African-American coaches in the National Football League. Even with the first black president and African-Americans and Hispanics wielding more power than ever in Congress, there are just two Senate chiefs of staff who are minorities. In the lower chamber, there are only five white lawmakers who have African-American...