What Heath is alluding to here is the “Overton Window,” an invention of the US conservative right. The “Overton Window” refers to the political ideas that are seen as politically acceptable, palatable, mainstream, centre-ground, and so on, at any given time. The ideas outside the Window are seen as extreme, fringe, deluded, ridiculous.
“Owen Jones: if Jeremy Corbyn wins, prepare for a firestorm” at