What impact do the small political parties, like the Green Party, and the Libertarian Party, have on the two major political parties in the United States?

I live in Canada and we have many political parties that form coalitions to get things done. They have a use here in Canada like with issues of Health Care, the environment and the right to carry guns. In Canada's recent election, a coalition of the NDP, Green party and Bloc Quebecois prevented Stephen Harper's conservative government from gaining a majority in Parliament. So, the smaller political parties do have power.

What about the political parties in the U.S. other then the Democrat and Republican parties? Could they use they use their power this way under the U.S. political system?