Overton

Please look at the article on Wikipedia, from which this is lifted https://en.wikipedia.org/wiki/Overton_window

The Overton window is the range of policies politically acceptable to the mainstream population at a given time.[1] It is also known as the window of discourse. The term is named after Joseph P. Overton, who stated that an idea’s political viability depends mainly on whether it falls within this range, rather than on politicians’ individual preferences.[2][3] According to Overton, the window frames the range of policies that a politician can recommend without appearing too extreme to gain or keep public office given the climate of public opinion at that time.

A new idea fills the window of what the public regards as unthinkable, causing the desired idea to shift into the window of what the public views as sensible, without its proponents necessarily having explained any benefits of the desired idea.