What noun could I use? "Hotbed"? "Hotspot"? Please don't mention the expression "safe state".
The city was hardly a liberal __________: the conservatives have won five consecutive elections there
You can use stronghold figuratively which is also prevalent in political context.
figurative and in figurative contexts. Esp.: a place where a particular cause or belief is strongly defended or upheld. - OED
Here is a very similar usage I've found from a prominent newspaper, Toronto Sun, published in Toronto, Canada:
As a native Hamiltonian, let me tell you that the Liberals are faltering in Steeltown. The city was a Liberal stronghold for years, the base of John Munro and Sheila Copps, and now it’s mostly represented by the NDP and Conservatives.
"Hotbed" could work. Also consider "bastion", which is often used to describe a place in which certain political sentiments are strongly held:
- anything seen as preserving or protecting some quality, condition, etc.
a bastion of solitude
a bastion of democracy
3: center, focus
The bookstore has become something of a nexus for the downtown neighborhood.
This term is value-neutral, without negative or positive connotations. But it has more of a sense of things coming together at a place; Merriam-Webster is correct with its "focus".
If you want to suggest that a certain place is both a center of and a source of specific ideas, you can call it a hub.
Territory describes "an area that an animal or group of animals uses and defends"; in a somewhat metaphorical use one can use it for areas controlled by tribes, including virtual ones like political factions.
In an almost antithetical use of the word one could, in ironic exaggeration, say that the city is not exactly "liberal heartland". It's antithetical because heartland is typically used to for "the central geographical region of the U.S. in which mainstream or traditional values predominate", and not liberal ones.
If the city is surrounded by conservative rural places, as is often the case, one could also consider it an enclave ("a distinct territorial, cultural, or social unit enclosed within or as if within foreign territory") if it were liberal.
Since it is, probably against expectations, not liberal, it is "hardly a liberal enclave".
The word "Bubble" would work well:
An enclosed or isolated sphere of experience or activity in which the like-minded members of a homogeneous community support and reinforce their shared opinions - Merriam-Webster
A situation in which you only experience things that you expect or find easy to deal with, for example opinions you agree with, or people who are similar to you - Cambridge Dictionary
[possessive 's'] paradise
an ideal or idyllic place or state. "the surrounding countryside is a walker['s paradise]"
"The city was hardly a place with liberal ideology: the conservatives have won five consecutive elections there."
"Ideological is an adjective that describes political, cultural, or religious beliefs. An ideology is a body of ideas, and those who agree with the main idea of something take an ideological stand to support it." – https://www.vocabulary.com/dictionary/ideological
"Sanctuary city (French: ville sanctuaire; Spanish: ciudad santuario) refers to municipal jurisdictions, typically in North America, that limit their cooperation with the national government's effort to enforce immigration law."
"The city was hardly a liberal sanctuary; the conservatives have won five consecutive elections there."
[possessive 's'] wet dream
informal. something especially appealing to a particular type of person
"hardly a liberal's wet dream"