The Pakistani ISI is one of the most capable covert services in the region. They'd have no need of US help, which would not be particularly necessary. Interestingly enough, I was listening to the radio on the way home. They happened to be playing several interviews with Bhutto; in one she actually mentioned the threat of the ISI. Too bad that she didn't look out for her own security enough. She also mentioned her dislike of the USA's support of Musharaf alongside her condemnation of terrorism.
Had the assasination attempt somehow failed, however, the US would likely do everything in its covert power to ensure that Bhutto was never elected. Bush was not keen on several of Bhutto's policies, namely granting more independence to the tribal areas on the Afghan border and "destabilizing" the country with democracy. Much preferred is the Musharaf way of keeping a tight grip on the judiciary and government religious institutions.