backbencher

The junior members in the British House of Commons who occupy the back benches of Parliament, sitting behind party leaders and top government officials.

BBC: “Backbenchers are also sometimes known as private members and thus a backbencher can introduce an original idea for legislation in the form of a Private Member’s Bill. Backbenchers have more freedom to speak as they are not as constrained by loyalty to the government. This can also pose problems for the party whips who try to impose party discipline.”

The term has also come to refer to the rank-and-file members of the U.S. Congress who are not part of their party’s leadership.