I think it's more the latter than the former. Political identities crystallize around issues, or sometimes around other identities. The two hot-button issues in Finland right now are immigration and nuclear power. The former is directly about identity, and the latter is *the* defining issue for one major part of the political spectrum -- the Greens started out as an anti-nuclear-power movement, and while they're enormously heterogenous otherwise, this is one thing that they're (near)unanimous about.
That means that when someone attacks your politics, it feels much the same as if he attacked your ethnicity, language, gender, nationality, religion, or any other building block of identity.
I don't think it's a coincidence that political rhetoric is the most heated in countries with long traditions of fixed and inherited political identity -- in the US, party affiliation is inherited about as strongly as religion, for example, which makes the schism between Republicans and Democrats as deep as the schism between Shi'ites and Sunnis in the Muslim world. France is very similar, although being a multi-party system, it's fragmented instead of polarized.