"write off" when used like this is functionally equivalent to "dismiss", i.e. to stop caring about it. "write off" it comes from the financial world, usually about taxes. a "write-off" is something that you claim as a "business expense", (or some other kind of expense which does not require taxes paid), so you don't have to pay taxes on it.
the sentence is not very clear: I will add some marks to help you separate the parts:
That was on Wednesday, November 20, 1963, the second of his last two full days in Washington.
Anyone [(who is) under the impression that the President of the United States was devoting his full energy to wardrobes and coiffures] is invited to examine his (the President's) activity during those final White House hours. (意譯:Anyone who thinks that the present is only thinking about clothing or hairstyle should see what he does during those final hours)
Monday had been a day of formal speeches and informal politicking in Florida (another ten electoral votes that couldn't be written off [we need those 10 votes, and we are unsure we will win them]), and Tuesday he had reached the office early. 】