Final answer:
Third parties have influenced U.S. elections by presenting new positions that major parties later adopt, as seen with the Progressive Party in 1912, which affected the election outcome and policy discussions.
Step-by-step explanation:
Third parties have influenced elections in U.S. history primarily by presenting positions which the major parties then adopt. While third parties have rarely won the presidency and do not often outpoll the major parties, they play a crucial role in shaping political discourse. By championing specific issues, third parties can attract attention and force the dominant parties to address these concerns within their platforms. An example of this influence is the Progressive Party, which in 1912 drew enough votes away from the Republicans to help the Democrats win the presidency. Overall, while third parties may act as 'spoilers', they also contribute to the evolution of policy and party positions over time.