Two-party system
Third parties, meaning a party other than one of the two dominant parties, are possible in two-party systems, but they are often unlikely to exert much influence by gaining control of legislatures or by winning elections. While there are occasional opinions in the media expressed about the possibility of third parties emerging in the United States, for example, political insiders such as the 1980 presidential candidate John Anderson think the chances of one appearing in the early twenty-first century is remote. A report in "The Guardian" suggested that American politics has been "stuck in a two-way fight between Republicans and Democrats" since the Civil War, and that third-party runs had little meaningful success.
Party switching in the United States
A second reason is to gain power and influence. The incumbent may be a member of the minority party in a legislature and would like to gain the advantages of being in the majority party, such as the potential to chair a committee. A disaffected incumbent who might not hold a leadership position or feels ignored or mistreated by the majority party might join the minority party with the expectation of holding a leadership position in the minority party and if currently elected, having the complete support of the minority party for re-election, who would certainly want to have more elected officials in their ranks.