be pushing at an open door

be pushing at an open door

To be attempting to achieve a desired outcome that is already ensured of success, typically due to being widely accepted or supported. Primarily heard in UK. It turns out they were pushing at an open door with this legislation, which polls show as having an 80 percent approval rate.
See also: door, open, pushing
Farlex Dictionary of Idioms.

be pushing at an open door

or

be pushing against an open door

BRITISH
If someone is pushing at an open door or is pushing against an open door, they are finding it very easy to achieve their aims. Investors in slave ships had better places to put their money. Reformers, in effect, were pushing at an open door. Congress will give him what he wants. It's like pushing against an open door.
See also: door, open, pushing
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • box clever
  • boxing
  • be boxing clever
  • chance (one's) arm
  • chance your arm
  • You can’t win ’em all
  • you can't win 'em all
  • you can't win them all
  • (you) can't win them all
  • can’t win em all
References in periodicals archive
Posner's jeremiad has by now been reviewed to death, and pointing out the book's inanities would be pushing at an open door. Perhaps better would be to ask how it is that Public Intellectuals elicited Op-Eds nationwide.