battle bus

noun1

1. •
   (UK, informal) A bus that is used as a mobile office and publicity centre by a politician or party during the run-up to an election

   • [Photo shows Steel sitting on a coach emblazoned with "DAVID STEEL'S LIBERAL BATTLE BUS". Photo caption reads:] STEEL'S ON WHEELS: Setting off from the National Liberal Club in his battle bus. — 1979