Pulled from the comments on Alex

The Bartels result may be just showing that in an economy when
average incomes are are rising rapidly, the low income groups benefit
more than the higher income groups.  Since WWII, with the exception of
Eisenhower, no Republican was president when the average income was
rising rapidly.

Here is the link for a relevant graph.  Here is a graph of the Bartels result.  And here is Greg Mankiw on inequality and unions, in case you missed it, perhaps Greg’s best post so far.