Final answer:
About two-thirds (66.7%) of U.S. states are dominated by a single political party, with many local areas also dominated by one party despite a federal two-party system mainly consisting of Democrats and Republicans.
Step-by-step explanation:
About two-thirds of the states in the United States are dominated by a single political party, and even in states with a strong two-party system, there are many local areas dominated by a single party. The two-party system in the United States, comprised of Democrats and Republicans, includes minor or third parties but is primarily dominated by these two due to the winner-take-all Electoral College system and ballot access rules. States like Japan and the United Kingdom have also seen dominant party systems, although the dynamics differ, suggesting that a variety of factors contribute to the dominance of political parties within individual states.
As of early 2021, Republican majorities were present in the legislative houses of thirty states, while Democrats controlled eighteen. Only in Minnesota was control split between the two parties. The two-party dominance extends beyond electoral success, also shaping the party coalitions and the electorate, with a notable number of Americans identifying as independents despite the prevalence of the two major parties.