From Nixon vs. McGovern to Trump vs. Clinton

The 2018 General Social Survey is out.  Since the GSS is given every two years in the early part of the year, 2018 is the first one since the election of Donald Trump.  The GSS contains questions on whether you voted in the most recent presidential election and, if you did, who you voted for.  It also has a number of questions which have been included since the 1970s, making it possible to look at changes in the the association between opinions and voting choices over a long period of time.  I considered four opinions:
1.  Some people think that the government in Washington ought
to reduce the income differences between the rich and the poor,
perhaps by raising the taxes of wealthy families or by giving
income assistance to the poor. Others think that the government
should not concern itself with reducing this income difference
between the rich and the poor. [Place yourself on 7 point scale]
2.  In general, some people think that it is
the responsibility of the government in Washington to see to it
that people have help in paying for doctors and hospital bills;
they are at point 1. Others think that these matters are not the
responsibility of the federal government and that people should
take care of these things themselves; they are at point 5.  [Place yourself]
3.  Some people think that (blacks/negroes/African-Americans) have been discriminated
against for so long that the government has a special obligation
to help improve their living standards; they are at point 1.
Others believe that the government should not be giving special
treatment to (blacks/negroes/African-Americans); they are at
point 5. [Place yourself]
4.  Please tell me whether or not you think it should be
possible for a pregnant woman to obtain a legal abortion if:
The woman wants it for any reason.

I computed the correlation of opinions on each with vote for the Democratic or Republican candidate (third party voters were excluded) in elections from 1972 to 2016. 

The correlations for equalization, helping blacks, and helping with medical bills track each other very closely--that wasn't why I picked them, it just turned out that way.  In the 1970s, opinions on abortion had almost no correlation with vote, but then it started to catch up and it's now comparable to the others.  There's an upward trend for all of them--that is, supporters of the Democratic and Republican candidates are more distinct.  People have known about the increasing association between opinion and party for long time, but some new things emerge from a closer look.  First, there is a difference between the trends for abortion and the other issues.  It's close to a linear trend for abortion, but not for the others.

I took an average of the correlations for the other three items and plotted it against time.  There's clearly a more rapid increase in the 21st century.  It's hard to date it exactly, but there was a large increase between the 2000 and 2004 elections, and another one between the 2004 and 2008 elections.  In 2016, the correlations for general equality and helping with medical costs declined slightly, and the correlation with helping blacks rose.  Those changes fit with common accounts of Trump's appeal, but they were not very large.

Second, there has not been a shift from emphasis on "economic" issues (the first two) to "social" issues (abortion and arguably race), which people often say has happened.  "Economic" issues are less important in a relative sense because some new social issues have emerged (just abortion here, but I would guess that some others have followed a similar course), but more important in an absolute sense--that is, vote can be predicted more accurately from economic opinions than it could back in the 1970s.   

Third, there has not been much variation between individual elections, suggesting that specific characteristics of the candidates or campaigns have not have much impact. 

PS:  The correlations for 1996 and especially 1992 are probably exaggerated because of votes for Ross Perot.  I expect that Perot voters were mostly in the middle ideologically, making the separation between Democratic and Republican voters bigger.  I don't think that John Anderson had much impact in 1980--the opinions of his voters were probably close to those of Carter's.