Electronic states associated with the low-lying Born-Oppenheimer spectrum and crucial for the stability and experimental observability of the doubly positively charged carbon system C2+2 are studied by ab initio methods. This includes computation of potential energy curves and transition moments by multireference configuration interaction methods, and an investigation of the corresponding vibrational resonance levels and lifetimes. By analogy with the electronic ground state X3Š-g of the isoelectronic neutral molecule B2, the lowest potential energy curve of C2+2 that supports quasibound vibronic motion belongs to the state 1 3Š-g. In case of C2+2, however, this state is destabilized by a crossing with the repulsive potential energy curve of 1 3Ýu, and the induced electronic transitions represent the major decay channel of C2+2 (1 3Š-g). Also the quintet state 1 5Š-u is quasibound; whereas most of its vibronic levels are practically stable against dissociative tunneling, interactions with other electronic states furnish the principal decay mechanism for 1 5Š-u. Additional bonding and stability properties of C2+2 are exposed by monitoring the behaviour of potential energy curves while rising the nuclear charge from the neutral to the doubly positively charged situation.