When some knowledge is available to most of us, quite evident and widely accepted, we say it's 'common knowledge'.
Let's think about the same situation, except that this so-called knowledge is proven wrong, unfounded from the very beginning, or simple dogma.
'Common rhetoric' seems more like it: things people say all the time despite being wrong. But it implies that there's an outspoken support of those ideas and goes beyond the scope of the expression I'm after.
What should I call that 'common mindset', that erroneous 'usual way of thinking'?
-I'm reluctant to ask Sally to prom.
-And I can see why; the common rhetoric is you should go with Janet.
The common rhetoric in the GOP is that tax cuts for the rich stimulate the economy so much that they pay for themselves. This unfounded, nonsensical claim has received the appropriate name of voodoo economics.