Neither the progressive populist faction nor the neoliberal moderate faction of the Democratic Party is making the mistake you charge; he Progressive populist faction was aimed at the problem before the Trump symptom had even fully manifested (not because it was fueling nationalism, but because they were concerned with the same underlying problems independently); the (decreasingly, but still) dominant neoliberal faction was certainly in denial about it until too late to prevent Trump, but when Hillary Clinton, as part of a discussion series on the rise of right-wing nationalist populism in Europe and the Americas says that Europe needs to curb immigration to cut off the fuel for nationalist populism, she's clearly focussed on the problem, whatever you think of her proposed solution.