play first fiddle

To have a leading, commanding, or controlling role, position, or part (in something). (A less common extension of the phrase "play second fiddle," meaning to serve in a subordinate role or position.) Ever since the power shift in Congress, Senator Smith has been playing first fiddle in the agenda for tax reforms. Though she's had many small parts in a variety of films, this is the first movie that sees Ms. Warren playing first fiddle.