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.
be pushing at an open door
or be pushing against an open door
BRITISHIf 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.