When the Supreme Court two years ago ruled that corporations could make unlimited political donations as an expression of free speech, the justices probably did not have in mind paper corporations operating in some cases from postal boxes. A super PAC supporting President Obama and another that is allied with the Tea Party draw funds from affiliated tax exempt “social welfare’’ advocacy organizations that are not required to disclose their donors. So far, it’s a relatively small amount of the cash flowing into super PACs, but in a few cases it is difficult or impossible to trace it back to its source. It’s called dark money, obscuring in these cases who is trying to influence federal elections and why.