Political donations should not be given back. Its a donation. You're not buying a service. It is no longer your money. If we're going to structure our rules around the idea that donors are owed something by their candidates, we might as well staple a price tag to the back of Mitch McConnell's suit. It would be more honest that way.
I'm sorry, I misunderstood your point and was trying to deal with the already-allowed option of handing back monies. That's only, at best, as good as investment as your candidate winning, juggling
your needs (however they see fit, along with everyone else's) into a plan of doing something legislative in your favour and then carrying out that plan in the face of all competing plans. If you overcontribute to him/her and (s)he wins, that money is diluted as everyone else is lobbied. If you overcontribute and the campaign fails, you got nothing at all. Getting a fraction of the donation back is sparse comfort if you realise you could have funded two or more candidates for the same position, and got more
elected persons willing to grant you favours. Not that I'm a fan of cash-for-influence, but given that campaign funds are a thing (not a commission-funded guaranteed five-minute TV slot per day, with identical production values, and evened-out prominance, behind each enrolled candidate's unique messages, and then a total ban on anything that involves personal costs like rallies or sky-writing planes) then campaign funds are a thing. Solving bribes-but-not-in-name is a separate thing. And bribe-money that pays for a retired failed-candiate's future holidays is probably the best kind of bribe. A wasted one that could have been spent better elsewhere, so lessening the impact of bribes overall.
And you missed that I said I generally favoured passing on unused funds forward to successors of the same ilk, the original person losing all future rights to them once so declared and therefore
no fund that pays for sea-food extravaganzas as a private citizen (if the new guy perhaps entertains the old one as a consultant, that's on
their tab, and part of
their funding limitations and accounting). Now
those bribes might well be more effective. But depends heavily upon the retiree passing on their funding to a person that entirely shares the same obligations of the retiree. Dilution of interest (intellectual, not financial) occurs as the retiree might have changed path away from the original donor (or vice-versa), or the recipient chosen by the retiree actually has their own ideas on this one, insignificant, issue and is Pro-Basketweaving where the original donor was attempting to influence an Anti-Basketweaving candidate, but still benefits from the lump of money because of his overwhelming similarity of opinions regarding the serving of cabbage in school canteens (an issue the donor has nomstrong opinion of, but retiree and newcomer both agree strongly enough to form such a proxy-donor relationship).
And, as said, It's Complicated. First of all Real Life, second of all Idealised Real Life, third of all The Unknown Tapestry Of The Future, where the importance of baskets and/or cabbage may rise or fall (and even repolarise) in the minds of many. Can't legislate for that,
at all.