Close states became rare in presidential elections

How many states were genuinely competitive, election by election? Each row is one state (or DC) in one presidential election from 1976 to 2016, with the Democratic margin in percentage points.

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The spread of margins widened

Each box is one election's 51 margins. The boxes get taller over time: states moved apart from each other, whichever party won nationally.

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Fewer states near the line

Each dot is one state in one election. The band within five points of a tie was crowded in 1976 and nearly empty in 2012. In 1976, 37.3% of states were decided by under five points. In 2012, 7.8% were. The closest of all sits on the line: Florida's 2000 margin was 0.0 points, and it decided the presidency.

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Utah and the District anchor the ends

Each cell is one state's margin in one election, blue for Democratic and red for Republican, on a scale fixed from 40 points either way. The District of Columbia's margin reached 86.4 points in 2016, and Utah's reached -52.2 in 1980. Down the middle, the pale rows of the 1970s give way to columns of saturated color: the same states, further apart.

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Takeaway

In 1976, nineteen states were close enough to swing. By 2012 there were four. Campaigns now fight over a handful of states because only a handful are close.

Limits

Margins are shares of all votes cast; third-party candidates widen or narrow them. 2016 had eleven close states, so the trend is not a straight line.