In the last few decades, Democrats have expanded their advantages in California and New York — states with huge urban centers that combined to give Clinton a 6 million vote edge, more than twice her national margin. But those two states elect only 4 percent of the Senate. Meanwhile, Republicans have made huge advances in small rural states — think Arkansas, North and South Dakota, Iowa, Louisiana, Montana and West Virginia — that wield disproportionate power in the upper chamber compared to their populations.
We can quantify the partisan bias of Congress over time by measuring the distance between each national presidential result and each year’s presidential result in the median House and Senate seats. So, in 2008, for example, Barack Obama won the popular vote by 7.3 percentage points, but Democrats won the median House seat by 4.4 points — a pro-GOP bias of 2.9 points.
Today, the pro-GOP biases in both chambers are at historic highs:Read more at TheFederalistpapers