Do they need to? I'd like to know who supports what he says other than the people who are already considered part of that fringe. I don't. I don't know anybody who does.
It should not be necessary for any group to distance itself from extremists if it's fairly self-evident that the extremists are not considered part of the mainstream. The best is to let lunatics be. If you get into a mudfight with a pig, only the pig has fun, right?
If people considered a fringe guy to be mainstream, then that's a different situation. Then I'd step up and clarify. But I don't see that happening.