I think Libertas's point is that "white ethnic" voters in 1928 and 1988 are really not comparable. In 1928 "white ethics"/Catholics were concentrated in ethnic areas, disproportionately lower-income, and discriminated against. They were also organised in ethnic urban machines. So, as you say, they voted for Al Smith for ethnic reasons. By 1988, those factors were irrelevant and most "white ethnics" voted on ideological rather than ethnic grounds. A Greek-American who voted for Dukakis would have been atypically conservative or largely apolitical.
