There are edge cases in this that concern me a little. Facts should be from reputable sources? Not quite. They should be from appropriate sources. The British National Party is not reputable and (like even some reputable British political parties) cannot be expected to publish accurate membership figures and certainly cannot be cited on issues such as social concerns of people living in Sheffield. It is a perfectly appropriate source, however, for the wording of a campaign address given by its by-election candidate in Sheffield, although it must be attributed if the text has not been published elsewhere. If one is writing about the British National Party's political strategy in South Yorkshire then appropriate reference to material published by the party itself (say the party's official mailshot delivered free by Royal Mail during the by-election or words obtained from the party's official website) may be used with caution, as long as material published solely by the party is not given undue weight and is treated as campaigning material and not a supporting source for factual claims made in the material.
Currently I'd feel confident in arguing for appropriate use of such material, but it may be that we want to employ higher standards and omit such material altogether.