The basic idea behind postmodernism is that if enough people agree on a certain belief, that is all that matters.
As Roger Donway explains in The Collapse of a Postmodern Corporation:
The philosophical essence of the postmodern, or anti-Enlightenment, outlook is that there exists no external reality to which our beliefs should conform. On the contrary, say postmodernists, the nature of reality simply is what people believe and say it is. Of course, people cannot believe and say anything they like. Their beliefs and speech must be coherent and consistent. And if they want to work with others, they must ensure that the group is in agreement about what to believe and say. But that is the goal: constructing a shared narrative that supports the group's desires and activities. So long as that is achieved, no "external reality" is going to come along to correct or punish them.
But global warming alarmists and progressives need to get the whole country to come together and work to prevent climate change or make a government run health care system work, or make it true that fiscal stimulus prevented another Great Depression. Since not everyone actually agrees on this, they need to agree at least among themselves that everyone agrees with them. So we have global warming alarmists insisting that there is a consensus, or Krugman followers claiming all sorts of their ideas are "mainstream". They need to come to agreement that there is an agreement that doesn't exist, in order to continue believing things that are not true.