No, they don't all have to be assumed. What needs to happen is something resembling their budget, on the order of plus or minus a few trillion dollars, is implemented.

At least $100 billion = the approximate marginal altruistic value of the "better" candidate. I think this is also very conservative. The annual federal budget is around $3 trillion right now, making $12 trillion over a 4-year term, and Barack Obama and Mitt Romney differ on trillions of dollars in their proposed budgets. It would be pretty strange to me if, given a perfect understanding of what they'd both do, I would only care altruistically about 100 billion of those dollars, marginally speaking.

ETA: However, even this is unlikely to be entirely affected by the outcome of the presidential election, as this depends mostly on Congress.