There are definitely some people who we shouldn't even bother trying with. They're unlikely to move anyway, and even if they do, the effort spent moving them would be better spent moving many others.

The risk is putting so many people in that "don't even bother" category that we defeat ourselves from the jump. This is one reason I dislike the "social democracy is the moderate wing of fascism" meme (that notably even Stalin eventually rejected), as well as a major problem with the Settlers thesis -- if you write off the vast majority of politically active people right away, all you've done is create a recipe to accomplish nothing and feel right about it, i.e., defeatism with liberal characteristics.