[James]

I'd have thought you'd play to the middle ground if the other side is moving further to the extremes. Otherwise (in a two-party system) a lot of the public in the political middle don't feel their views are represented.

I don't know a lot about US politics but the Democrats might think they can put anyone up against Trump, and they will still win - so take the chance to nominate a radical. That might not work out for them though.