Third parties have always existed in the United States. They typically represent interests that are already voiced by the two major parties. However, they have had little influence on ideas and elections. Usually, they do not become permanent by replacing a major party.

Answer: Third parties in the United States are political parties other than the two major ones, the Democratic and Republican parties. While it’s true that the two-party system has dominated U.S. politics, third parties have had various levels of influence. They often bring attention to issues that the major parties may prefer to ignore or give minor emphasis. Nonetheless, their impact on ideas and elections can be significant even if it is not always direct or immediately evident.

Third parties have faced multiple obstacles in gaining traction in the US political landscape. Major challenges for third parties include:

1. Electoral system: The "winner-takes-all" system in the U.S. often discourages voting for third-party candidates because such votes are seen as potentially 'wasted'. This system differs from proportional representation systems found in many other countries, which can allow smaller parties to gain legislative seats according to their proportion of the vote. 2. Debate inclusion: Third-party candidates often struggle to meet the criteria to be included in debates, limiting their visibility to voters.

3. Ballot access laws: The rules for getting on the ballot can be onerous and vary from state to state, making national campaigns particularly challenging for third parties.

4. Campaign finance: Major parties have more extensive and established fundraising networks, often leaving third parties with fewer resources.

5. Media coverage: Media outlets tend to focus on the candidates from the two major parties, resulting in less coverage of third-party candidates.

Even with these challenges, third parties can influence major parties to adopt some of their ideas or positions on specific issues. Occasionally, when a third party gains a significant following, one or both of the major parties may shift their policies to attract those voters. In some rare cases, such as the rise of the Republican Party in the 1850s, a third party may even come to replace one of the existing major parties, although this is historically uncommon and has not occurred since.