Superform Approach to Equivariant Localization in Supergravity

We identify a superspace mechanism behind equivariant localization in supergravity. We show that closed superforms generate, on supersymmetric backgrounds, equivariantly closed polyforms. After presenting the general mechanism, we construct such polyforms for vector and linear multiplets, and for chiral and BF action principles, in off-shell $4d$ $\mathcal{N}=2$ conformal supergravity, reproducing and extending recent results. Our construction provides a geometric first step toward equivariant localization of BPS observables in supergravity, including higher-derivative theories, and holography.