The white supremacists who marched in Charlottesville are members of the Reactionary faction of the GOP. As such, they are an important part of the base, not just for Trump, but the entire Republican Party.
They have an uneasy relationship with the other factions. Christian Democrats, of course, genuinely abhor them. PBPs find them embarrassing, but use them as lobby fodder for the things that really matter–tax cuts and deregulation. In a similar vein, the Conservative Libertarians use them to campaign for reductions in the size of the state.
PBPs in particular have become very good at attracting Reactionary votes without looking like racists to the general public. The difference between Trump and the average GOP politician is that he doesn’t care about the optics, and he genuinely sympathizes with the Reactionary agenda. The Reactionaries, for their part, are clearly tired of providing votes for tax cuts for the wealthy without receiving anything in return. Hence, the conspicuous divisions in the GOP that are largely responsible for the lack of progress on legislative issues.