A self‐adjoint, gauge invariant stability functional &dgr;WTGfor determination of a sufficient condition of tearing mode stability is obtained for a plasma with a uniform pressure. The plasma is stable if there is no free energy available to support energy flux into the singular layer. The &Dgr;’concept, valid in simple geometries, is generalized to reflect variation of equilibrium quantities on the flux surface. The relationship of &dgr;WTGto the ideal stability functional and other possible forms of tearing mode stability functionals is clarified through gauge transformations.