From my paper with Delia:
Party identification and self-defined liberalism/conservatism are increasingly correlated with positions on specific issues. The increases in correlations have been highest for moral issues. Issue positions have also become increasingly correlated with each other–but the increases have been smaller than the increased correlations with party ID and liberal/conservative ideology. Correlations between pairs of issues have increased by about 2% per decade, on average, while correlations of issues with party or ideology have increased by about 5% per decade (again, on average). The data come from the National Election Study.
Our story: voters are sorting themselves into parties and ideologies based on their issue attitudes; having done this sorting, they are aligning themselves slightly with their new allies.
the same thing has come to pass in the netherlands, according to van der Eijk & Niemoller, among others, but it has always been explained from the demise of the dutch "pillars" i.e. tightly knit social groups: catholics, protestants, socialists, and others. (in the old days group membership explained everything, now it is ideology, based on issues)
So it's interesting that it also happened in the US.
I would guess that in Europe the 'stance on morals' line would be a ' stance on immigration' line!