Swing vote

Imagine you're playing with your friends on a playground, and you all have to decide which game to play next. If two of your friends want to play tag, and two want to play red rover, you're the only one left who hasn't made up their mind yet. You are the swing vote because your decision will be the one that picks which game everyone plays next.

The same thing can happen when grown-ups vote for important things, like who will be the president of a country. Sometimes, there are two main people running for president, and their supporters are about equal in number. But there are also a group of people who haven't decided yet--they're like you on the playground. They could choose to vote for one person or the other, and whichever person they choose will win. Those people who haven't decided yet are called the swing vote because they "swing" the election one way or the other.