This question is asking if any politician has endorsed proportional representation. That is good scoping - the OP is asking about a topic, and seeking to elicit who has advocated it.
The problem is that "politician" can be fuzzy.
- Is the President or a Member of Congress / Parliament a politician? Sure
- Is the junior Soil and Water Board member of Bumpass, Va a politician? Um, maybe, but probably not what you care about.
- What about the Green Party? Well, definitely in Germany - but what about in the U.S.? or Russia? And what about the Official Monster Raving Lunatic Party?
- Is Bill O'Reilly or Stephen Colbert a Politician? Well, um, maybe. Depends
- Is a major candidate who lost an election a politician? Well, um...
- Is that cook who wrote up a conspiracy website a politician? Well, um, again...
How do we retain a level of scoping when seeking to ask "What politicians support X?" that preserves the intent of notability without making the term meaningless? Can we draw a clear line?