Summarize the three main effects of third parties on american politics.

At the national level, our "winner-take-all" elections mean that third parties are essentially blocked in any meaningful competition for power. That makes third parties functionally irrelevant, and factions within the Democratic Party and the Republican Party rivaling by influence within their margins both accumulating public support and getting the election of candidates who are on their side.  

That means that majority parties essentially function more as a coalition than as a cohesive entity, encompassing everyone from the most moderate to the most radical factions. Party leaders are elected by a small group of members and more than impose ideological lines establish bureaucratic rules and functions, at least officially.

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