relative strength of political parties; distribution of parliamentary seats
Used in political analysis to describe how many seats each party holds in a legislature, or their relative influence.
Looking at the distribution of seats after the election, the ruling party maintains a majority.
The relative strength of parties in the Diet could change significantly in the next election.
議席数 refers simply to the number of seats a party holds, while 勢力分野 emphasizes the comparative distribution or balance of power among parties.
勢力図 is a more general term for a map or diagram of power distribution, not limited to parliamentary seats.
Compound of 勢力 (power, influence) and 分野 (field, area). The exact historical derivation is uncertain; the term is used in modern political contexts.