Question

What impact have third parties had on the United States?

Answer 1

Some have been able to influence legislation.

http://www.thisnation.com/question/042.html

Answer 2

would this be the answer
the Democrats and Republicans dominate the U.S. political system.
many timesa third parties have greatly influenced national politics, even though their candidate did not win the election.

Answer 3

I suggest you be more specific about the impact of third parties.

However, if your teacher is just expecting a two-sentence answer, then your answer is acceptable.