I don't think bi-polar would disqualify anyone from holding office if they have a solid psychitriatric support system behind them - monitoring by a good psychiatrist including access to all MDs, good therapist and the right combination of meds CAN work out.
In Kennedy's case tho, he was clearly self medicating and probably not taking meds for his bi-polar at all. He's at a crisis point right now and it may take months to find a regimen that works... he may never find it.
He's self-medicating alright! The whole family is.