For better or worse, the major political conflict in the United States has never been (and is not now) between capital and labor. Instead, the major political conflict has been what was once called “the Slavery Question” and is now discussed in term of civil rights (although the civil rights movement has arguably always been just as or more focused on practical economic equality rather than simply de jure recognition of equality before the law.) To reframe this slight: the basic political conflict in the United States since its founding and through today concerns how the government should deal with what was once called white supremacy (and has now been re-branded several times) and its attendant traditional social and economic structures. Insofar as policies that further racial equality undermine the existing social hierarchy, voting for the party that seeks to promote racial and gender equality will promote better and more equal government policy for all citizens by eroding traditional power hierarchies (including traditional economic structures), even if that opportunity remains constrained by the modern capitalist economy. The question of much current economic structures can be disentangled from racial hierarchies is beyond my abilities, although there is no doubt a connection, as previously they were blatantly conjoined (i.e., the industrial agriculture economy of the South under slavery.)
As better described by Noam Chomsky and John Halle in their June, 2016 essay, the failure to advance the party most opposed to the current social power hierarchies results in “terrible suffering on the most vulnerable segments of society” and inhibits the institutional clout and organizational ability of the left to make further changes to the modern capitalist economy.
In light of the foregoing, I believe an answer emerges to the question of what party should I support, assuming both are equally dedicated to protecting the interests of capital against labor. That answer is: the party that most consistently supports racial equality.