"He's a whip on the Liberal side"

The Liberal Party, also known as the Whigs, was one of the two main political parties in nineteenth century Britain, the other being the Conservatives, or Tories. Broadly speaking, the Liberals stood for classical liberalism, supporting free market economics but also social and political reform, while the Tories defended the status quo, emphasising continuity and stability.

In a political party, a whip is a person responsible for party discipline, making sure that individual party members vote and act in accordance with party policy. The term comes from hunting, where 'whipping in' means keeping control of a pack of hounds, preventing any from straying.