[ptsneves]

Not dismissing your point, but you may be hitting at words that have ambiguity when translated. I am not a german speaker but would it be grammatically correct to say "Ich bin CDUer"? From my elementary school german it does not feel right.

On the other hand you can perfectly say "Ich bin christdemokrat" and here you are back to the American case with political support equaling identity.

An equivalently polarized country, Poland, does not have PiS supporters saying they are "Jestem PISowcy"(I am a Piser) or "Jestem POsy")(I am a POer). According you our theory I would expect Polish to identify themselves as written above.

In Portuguese, party names are also mostly abbreviations, and people will just say "sou do PS"(i am from PS/I am a PSer) for actual militants as well as supporters. This would hint at identity and support being the same. As a Portuguese i can tell you Portugal is outstandingly homogeneous and not polarized. This would falsify your theory as well as you would expect conflating support with identity would reflect polarization.

[stnikolauswagne]

I dont think anyone would ever say „Ich bin christemokrat“, a much more common saying (at least in the area where I live or the area I grew up) would be „Ich bin CDU-Wähler“/I am a CDU-Voter. This is coincidentally how my parents would describe their political leanings (SPD).

[Firadeoclus]

Certainly some people do say "Ich bin Christdemokrat", but that would usually be active party members, not just voters.

[rmbyrro]

In Brazil people say "I'm Petista" and they're extremely polarized, very similar to how the US looks like.

It's amazing that both sides' (right and left) ideas, at this point, lead to the same results in the long run, but they vehemently fight eachother like they were extreme opposites. Just because the short term presentation of the ideas is different.

[marax27]

Fair point, in Poland I wouldn't expect someone to call themself "PISowiec" ("PISowcy" is plural) - those terms are often used as derogatory. On the other hand, it's quite common to label either yourself or them as "prawicowiec/lewicowiec" (right-winger/left-winger).