OK, so there's quite a bit of gotcha political crap flying over the Ku Klux Klan, David Duke, et. al supporting Donald Trump. But that's all irrelevant.
What's relevant is whether or not he supports them.
Back in the day, I vociferously defended Ron Paul when people would try to put him down by noting that his weekly column appeared in David Duke's newsletter and in the Spotlight, a newspaper published by anti-semite and Holocaust denier Willis Carto. My view was simple: He put that column out for anyone to reprint as they pleased, and could hardly be held responsible if unsavory characters chose to take him up on the offer. It just didn't constitute evidence that he shared their ideas.
That position brought me in for later embarrassment when the Paul's old newsletters -- in which he applauded Duke and clearly pandered to the southern white racist demographic -- came to light during his 2008 presidential campaign. It turned out that he had been playing footsie with the bad guys. In my defense, I hadn't known that. And once I did know it, I stopped defending him.
Trump should get the same kind of colonoscopy. My guess is that it won't turn up any significant connections, for the simple reason that Trump has never needed racist money, while Paul seems to have relied on it to fund his return to Congress. But hey, all candidates bear close examination.