Why did the candidates spend the most in Florida, Virginia, and Ohio?

Those states are considered "swing states". This is where effectively both candidates are at a statistical tie in terms of the number of votes. The state could either swing toward red (republican) or blue (democrat). Also, historically swing states tend to be unpredictable and lead to various speculation as to what will happen on election day. This means that the states get more media coverage compared to other non-swing states. Non-swing states are where one party dominates another in terms of votes, so the party doesn't have to focus money/attention on trying to get out the vote. The outcome is already decided, so they move onto more important priorities.

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