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.