In the Cook Political Report vernacular, the classification of “Toss Up” is used to refer to races that are the most competitive of the cycle, and which either party stands a reasonable chance of winning. Even so, races that share the same ratings designation have many important distinctions from one another; in one state/CD, for example, the political demographics may benefit one party, while in another, the candidate/campaign quality favors a different party. As such, it’s easy to go down the list and assume that these races break somewhat evenly on Election Day.

In reality, however, the closest races in each election tend to break disproportionately toward one party. In looking back over the last twelve cycles, the races rated as Toss Ups in the House and Senate have split anywhere from 54 percent to 100 percent toward one party.

However, the most competitive House and Senate races don’t always break toward the same party in a given year.

For example, even as Republicans won 66% of the House Toss Up races in 2010, Democrats carried 71%

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