[partisanship / ideological shifts, etc.]
(I'd be interested to see the methodology behind the Economist graphic, if you have access to it.)
I think this misses a few key aspects, particularly through the definitions of partisan divide and which aspects of ideology are changing.
Unfortunately I don't think there's a newer version of this since 2016 - a version including the last 4 years would be quite interesting - but Pew Research took a look at trends on partisan gaps on a series of 10 items going back to 1992. (the image below goes to '94, but the
data itself goes back to '92.)
Where is the divide getting wider? Treatment of poor people; feelings regarding the government, immigrants, and African Americans; environmental laws; and the importance of military strength. Some topics - homesexuality, government spending, feelings on corporations - haven't changed as much. Notice that e.g. back in '94, liberals were as likely to think of immigrants as "a burden on our country" as conservatives.
Or, put another way, not every policy difference is wide, but the policies that are recently becoming the widest are also some of the most present in the public discourse. See, e.g.,
this more limited Pew research piece on racial and gender views comparing 2016 to 2020.
Considering all this, it'd be good to know
in what ways the most recent batches of congressional Dems are getting more liberal. It could simply be a matter of a wider range of perspectives present, seeing as there's
been a rather large bump of members who aren't white males over the past few terms.
Other factors in here are the generational divide (see e.g.
this Pew piece) and geographical self-sorting (this one is a more hotly-debated topic, though).
Self-identifying members of each of the two main parties do self-sort more than ever before, arguably largely thanks to an increased ability to do so in a broader / more free media landscape (i.e. the internet) - I'd argue that's not solely a leftist issue. Just as the past few decades have seen the rise of Huffington Post and other liberal outlets, so too has it seen the rise of Fox, OAN, Newsmax (at times the most popular news website in the US!), and others.