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