I have a basic recipe for white bread that is very dependable and
works great. I have tried to modify it by replacing about half the
white flour (I use Cargill "Hummer" high gluten flour) with whole
wheat flour, leaving everything else the same, and the results are
pretty much uniformly disappointing. The dough becomes sticky and
less elastic, and doesn't rise well, and the finished bread has a very
coarse crumb. I have tried increasing the amount of yeast, but this
doesn't seem to help. Is there a general principle to follow when
changing part of the flour in a bread recipe to whole wheat?

One problem may be that while it is easy enough to get hard wheat
white flour, the whole wheat flour which is readily available
(Hodgson, King Arthur) does not seem to be designated as true bread
flour.