Here are a few more questions on which it's possible to see if there's been a growth in extreme opinions. Gallup asked " In your opinion, is the death penalty imposed too often, about the right amount, or not often enough?" a number of times between 2001 and 2018. The results, as summarized on the Gallup website:
That looks like just a slight move against the death penalty, not a move towards the extremes. Gallup does not give a breakdown by party.
Then there's a question the GSS has asked since 2004: "Do you think the number of immigrants to America nowadays should be .... increased a lot, increased a little, remain the same as it is, reduced a little, reduced a lot. The means for Democrats and Republicans:
Democrats have moved in favor of immigration--on the average, they are somewhat in favor of increased immigration (zero means "remain the same"), while Republicans haven't changed much.
Now the standard deviation of opinions:
No trends--the one year that stands out is 2016, when the number of Republicans saying that immigration should be "reduced a little" increased at the expense of those saying that it should be should be reduced a lot and those saying that it should be kept the same or increased. That is, Democrats have shifted in favor of immigration, but haven't shown any special tendency to move to the "increased a lot" option.
Finally, between 1999 and 2019 Gallup asked about climate change: "From what you know about global climate change or global warming, which
of the following statements comes closest to your opinion?...Global
climate change has been established as a serious problem, and immediate
action is necessary, there is enough evidence that climate change is
taking place and some action should be taken, we don't know enough about
global climate change, and more research is necessary before we take
any actions, concern about global climate change is unwarranted." I couldn't get breakdowns by party for this one, but here is the overall mean:
Basically a movement towards regarding climate change as a serious problem. The standard deviation:
A pretty clear increase. The first time that the survey was taken (December 1999), 23% said that it had been established as a serious problem and 11% said that concern was unwarranted. The last time (December 2019), 44% said it had been established as a serious problem and but the share who said that concern was unwarranted remained at 11%. Support for the two middle options declined between 1999 and 2019.
So putting the results from this post and the last post together, polarization increased on two out of the five issues, didn't decline for any of them, and stayed about the same for three. In one of the cases where polarization has increased (abortion), it's roughly symmetrical--both Democrats and Republicans have moved towards extreme positions. It's hard to say for climate change, since we don't have a breakdown by party and there's a strong general trend which can plausibly be attributed to new information (average temperatures have continued to increase). Overall, there's no evidence for the general claim that Democrats have moved towards extreme positions more than Republicans.
[Data from the Roper Center for Public Opinion Research]