What is a swing state? why are they important

It is because US state where the two major political parties have similar levels of support among voters, viewed as important in determining the overall result of a presidential election.  The are important because  in American politics, the term swing state refers to any state that could reasonably be won by either the Democratic or Republican presidential candidate. These states are usually targeted by both major-party campaigns, especially in competitive elections. Hope this help you.

during the ming dynasty