No one who lacks knowledge of a subject is competent to pass judgment on that subject.
Conclusion
Only seasoned politicians are competent to judge whether a particular political policy is fair to all.
Evidence
(1) No one who lacks knowledge of a subject is competent to pass judgment on that subject. (2) Political know-how is not a matter of adhering to technical rules but of insight and style learned through apprenticeship and experience.
Evaluate
The argument's logic: only those with knowledge can judge; political know-how is acquired through apprenticeship and experience; therefore only seasoned politicians (who have that apprenticeship and experience) are competent to judge whether a policy is fair to all.
The slippery move is at the conclusion. "Fair to all" is a question about social impact and ethics — about how a policy affects different groups in society. The argument has shown that political know-how (the practical skill of doing politics) requires apprenticeship, but it never establishes that judging fairness requires that same know-how. The argument quietly equates "knowing how to do politics" with "understanding the social implications of policies" — and those aren't the same kind of knowledge. A philosopher, an economist, or an affected citizen might have plenty of expertise on policy fairness without ever apprenticing as a politician.
Goal
Find an answer that names this conflation: the argument equates political know-how (insight/style) with the kind of understanding needed to judge social implications and fairness.