What has been the consistent theme of the Democratic Party since it's founding?

[RINO Tom]

As others have said, a vague quest to change or preserve various parts of the economic environment in a way to help out the poor and working class, which is more often than not directly in contrast with business interests.

I'll also suggest that the Democrats have usually been more morally relaxed.  The GOP seems to have always fancied itself on playing the role of moral police.