In the title salt, 2C4H6ClN4 +·C4H2O4 2−, the complete fumarate dianion is generated by crystallographic inversion symmetry. The cation is essentially planar, with a maximum deviation of 0.018 (1) Å. In the anion, the carboxylate group is twisted slightly away from the attached plane, the dihedral angle between the carboxylate and (E)-but-2-ene planes being 12.78 (13)°. In the crystal, the protonated N atom and the 2-amino group of the cation are hydrogen bonded to the carboxylate O atoms of the anion via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. In addition, another type of R 2 2(8) motif is formed by centrosymmetrically related pyrimidinium cations via N—H⋯N hydrogen bonds. These two combined motifs form a heterotetramer. The crystal structure is further stabilized by stong N—H⋯O, N—H⋯Cl and weak C—H⋯O hydrogen bonds, resulting a three-dimensional network.
