We can then compare these district margins with the state as a whole to measure the median seat advantage. Since Clinton won Virginia itself by 5.4 percent, Trump’s 3.4-point edge in the median seat gives him an overall advantage of 8.8 percent.
That advantage is a useful number because it gives us an idea of how big of a margin the disfavored party would need to win by statewide to carry a majority of districts. Assuming all seats swung Democratic by the same amount, Clinton would need a statewide victory of nearly 9 points just to carry half the seats. Seats don’t usually all swing the same way in real life, but it’s still a helpful thought experiment.
This median seat advantage is particularly useful because it shows the built-in edge a party has. Democratic candidates running for House in 2012 won more votes than Republicans did nationally, but mainly thanks to gerrymandering, Republicans won a majority of seats. This was largely because Mitt Romney also won a majority of America’s congressional districts that year despite losing the national popular vote.
It just so happens that not only is Virginia’s 2nd the state’s median congressional district, but it’s also the country’s median, meaning Trump also carried a majority of districts in spite of the national popular vote. Trump had a national median seat advantage of 5.5 percent, which is a major reason why it will be so difficult for Democrats to win control of the House even if they win more votes.
You can think of that advantage as a 5.5 percent cushion Republicans have just from the maps. It might take a wave election similar to 2006 or 2008 for Democrats to regain the House thanks to these districts. If we want to ensure our electoral system rewards the party that wins the most votes with the most seats, reducing or eliminating the Republican median seat advantage would go a long way to doing so.