Minor parties have contributed MOST to US politics by?

Causing major parties to consider, and sometimes adopt, their ideas. Often, it takes a smaller party to put forward a new or potentially controversial idea-- it may not be a bad idea, but it may be perceived as controversial because it goes against what the majority party has espoused up to that point.

The two major parties (Republicans and Democrats) often tend to be very cautious when proposing new policies, for fear of alienating voters. They stick to their established talking points, and hesitate to violate the expectations of those who have supported them. But the minor or smaller parties feel they have less to lose, since the people who voted for their party did so with an expectation that they would be different and make bolder proposals. When a minor party introduces a new policy idea and gets it into the political discussion, there is the chance that, gradually, the proposal might gain some traction, to the point where it is ultimately adopted and implemented by a major party.