base of support; support base; supporting layer
Used in political contexts for a party's core supporters, and in technical contexts for a physical supporting layer or substrate.
The support base of this political party is mainly elderly people.
Gravel is used for the supporting layer of the road.
基盤 is a more general term for foundation or base, not specifically a supporting layer or political support base.
Compound of 支持 (support) and 層 (layer, stratum).