Overton window
Etymology
Named for Joseph Overton, an American policy analyst who in the 1990s described the window of politically acceptable positions a figure can hold without seeming extreme. The "window" is the framed opening through which mainstream opinion is visible at any moment.
noun · /OH-ver-tun WIN-doh/
The range of ideas the public is currently willing to consider acceptable.
What it names
Why yesterday’s unthinkable becomes today’s debate and tomorrow’s policy — the boundary of the "sayable" keeps moving.
Example
By stating the extreme version loudly, the campaign shifted the Overton window until the moderate version sounded reasonable.
Related idea
A dog whistle moves opinion quietly; shifting the Overton window moves the whole range of what can be said.