be pushing at an open door

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