Perhaps a better example is the UKIP party swaying the Tories to become more isolationist, and eventually triggering the Brexit referendum? A little bit different, knowing the political system of the UK enables smaller parties to be successful in a way the American system doesn't. But the principle is still the same. UKIP acting as a spoiler is factored into the strategy of the Tories policy makers, and thus they pivot their policies more in line with UKIP ideals to prevent bleeding voters to UKIP. A similar dynamic is at play on Canada's left-wing between the Liberals and the NDP

Brexit was about Tory backbenchers, not UKIP.

I'm not saying it happens in practice, I'm saying that's often the goal.

So you agree that it's a silly idea that doesn't actually work?

Doesn't work for what?