Chris below has an excellent comment:
Drawing the stacked-bars equally high when the total size of the groups they represent is substantially different creates a very misleading visual perception. I suggest redrawing with the y-axis “% of total population” – the 1990 stack will be significantly shorter, but if my calculations are correct, the Republican slice of it will be about the same.
Below I’ve redone the chart and standardized in reference to 1990 base population numbers. Since “Nones” have gone from 9 to 15%, and the American population has gone from ~250 million in 1990 and ~300 million in 2008, the total numbers give a different impression.
In absolute numbers the changes have been by multiples of:
1.24 – Republican
2 – Independent
2.52 – Democrat
2.2 – Other
In other words, the rise of the secular Left may be a function of the secularization of the Left. Let’s compare 1990 and 2008 in terms of politics & religion in the GSS. I had to increase the interval of the dates to increase sample size. Here are the proportion of those with “No Religion” in each party during the respective periods:
To my surprise , in both parties and among independents there is an increase in the proportion who have “No Religion, by about a factor of 2 (this is true even limiting to the sample 18-35, this isn’t just a function older secular Republicans who won’t drop their party affiliation).
Let’s flip this around and compare what proportion of White Protestants in both parties during these two periods where Fundamentalists.
The proportional change here is modest. Of White Protestants there was some shift among Democrats way from Fundamentalism, and one of Republicans toward Fundamentalism. To some extent I think this is probably the realignment which happened among Southern Whites, as they finally moved from being “Reagan Democrats” to post-1994 Republicans. But these charts do not highlight another point: the proportion of Protestants among whites in the Democratic party went from 55% to 42%, while among Republicans it changed far less, from 67.5% to 63%. The WASP New England Republicans who became Democrats in the 1990s were likely the most liberal and secular of New England WASPs, and so did not bolster the segment of White Protestants among Democrats, while Southern Whites most assuredly did for Republicans.