No, and they hardly EVER do. Even when a candidate changes his political party, as Charlie Crist did in Florida, he can legally keep the campaign funds that were donated to him - even though that isn't at all ethical.
The most common way candidates get funds is from Political Action Committees (PACs). PACs, created by the 1974 campaign finance reforms, can be created by a corporation, union, or other form of...
Because whatever/whoever you are investing in is not a sure thing. If you had invested in Enron 10 years ago, it was a bad risk. If you had invested in Microsoft 25 years ago, you'd be a happy camper...