Purity spirals are a social dynamic where members of a group compete to demonstrate ever-increasing levels of ideological purity. Moderate perspectives are seen as dissenters and are shunned as the group becomes more radicalised.
It's a big problem for the left because numerically they are now the minority of voters, but are still trending away from the center, and shedding their more moderate members.
We know it's not as much of a problem on the right, because the rate of party switching is still far lower on the right than the left, by a factor of about 1:4.
On the right, you can be shunned by MAGA or shunned by the religious right, but still be welcome within the republican party.
On the left, you cannot challenge with any of the tent pole policies without being shunned entirely.
I think YMMV here based on who you're around. If you're right-leaning in an urban setting or a coastal city, you'll find a lot of heterodox views. If you live in the Rural South or Rural Midwest and do not almost literally worship Donald Trump, you are shunned.
The opposite is true for the left and the Democratic Party. Those circles tend to be very orthodox if you live in a lefty urban area. Disagree with one major platform issue and you're immediately suspected. If you're in the suburbs or a rural area you'll find that left-leaning people are much more heterodox.
Purity spirals are most intense in enclaves, which are echo chambers.
You'll find the same phenomenon online with regard to echo chambers. If you're in a left or right wing echo chamber, the purity spiral phenomenon is intense.
It's a big problem for the left because numerically they are now the minority of voters, but are still trending away from the center, and shedding their more moderate members.