You've decided on who you want to support in the upcoming election. Bob seems like he would be exactly what the country needs. So you decide to donate to his campaign.
Then Bob goes off the reservation on you. He starts saying things you don't like. He starts supporting positions you don't like. You don't want to vote for Bob anymore-- oh my God, but you gave Bob money! You want a refund! Bob, give me my money back!
Ha ha, yeah, don't count on that happening.
Only rarely does a candidate refund money to a donor. If they do, it will almost certainly be for reasons of their own, most likely because they don't want to be associated with whoever the donor is affiliated with. Once you donate, you should pretty much consider the money gone for good.
Why would a candidate not give money back? Well, obviously the possibility exists that the candidate is unscrupulous enough to not care what you think as long as he's got your money. But there are other reasons.
*The candidate really does think it's theirs.
Which it is; you did give it to them.
*The money's already been spent.
This is by far the most likely reason you won't get your money back. You've heard how much it costs to run a campaign these days. You also have likely heard about how candidates end up in debt for some time after a campaign. It's not hard to connect the dots. Campaigns under competition are under immense pressure to use the entire warchest- and beyond- lest they get outspent and, subsequently, lose. The speed at which you spend over a given period of time is called your burn rate, and a burn rate only covers in-house expenses: salaries, taxes, fundraising talks. Any money left over after you're done burning goes towards advertising and voter outreach.
Many campaigns have such a burn rate that they have to take on debt, or inject money out of pocket, in order to conduct any actual campaigning. Since the whole point of a campaign is to, well, campaign, candidates often end up paying it off years after the campaign has ended. Hillary Clinton, for example, is still trying to settle her debt from her 2008 Presidential run. This can take a while, as who wants to donate to a campaign that has already lost? And in Hillary's case, it's doubly difficult, as high-salaried campaign advisor Mark Penn is viewed by Hillary supporters as a key factor in why she lost in the first place. The remaining debt would end up in the hands of Penn's legal firm, Penn Schoen Berland, and that's an extremely unattractive sell.
Not even safe campaigns are entirely safe; these politicians stockpile money in their off years, like a squirrel storing acorns for winter, and anyone willing to hand over some of their warchest to help out a partymate in need will usually end up in an improved position within the party during the next legislative session. Take this link, which shows a series of posts from the 2008 season pretty explicitly linking donations from personal warchests to the Democratic Congressional Campaign Committee, to the prospect of getting to vice-chair said committee.
*The candidate thinks that it pretty much serves you right.
This is the camp I fall into. While you really should be voting regardless of the field- even if it's a lesser-of-two-evils choice- the decision to donate money is yours and yours alone. Who to donate to, how much to give, and when to give it. You've essentially bet money that that particular candidate will be the best for you and the country. If that bet goes sour, that's just too bad. If you came to like the opposition, why did you not choose better when you donated? If they changed their positions since your donation, why did you not wait until later in the campaign? If new information was introduced about them, why did you assume this information, or something like it, would not be introduced?
Which leads to a curious phenomenon. On the rare occasion that you do get the money back, considering all that... perhaps that's a signal that the candidate you just took your money back from is worth your vote after all.