This cause should ultimately be left to the donator to decide. Campaigns are like a non-profit in that they do not sell products or services to make some extra change. When a private citizen decides to put money towards a campaign, it is a voluntary expense — a donation. The campaign can do whatever it wants to advance its goals, similar to how we typically perceive donations to the Red Cross. If people are not happy with how the money is retained or spent, try to take it to court or simply don’t donate.
Since public organizations must disclose their finances, campaigns, which are of a similar nature, should be doing the same, thus allowing donators/voters the opportunity to see how money is being spent. Any donations used for personal use should be considered as taxable income. Expenditures that are written off as business items but are seemingly personal can be evaluated by the donator independently. The campaign will be “punished” by a loss of a vote or gain of a public critic if there is any funny business going on.
Plus, for candidates that anticipate multiple runs, left over cash can give them a boost. It is arguable that it is unfair, and maybe it is to an extent, but it can also help legitimate candidates maintain their momentum. If an incumbent did not anger their people enough to not get elected again, then the extra cash might not have helped much anyways.
Lastly, if the money is to be given off to charities, who decides where it should go? As a candidate, can it go to my own foundation that supports other political issues? The complex answers suggest we probably need to think about that more.