The source of this discrepancy is that Democrats will win their seats with big majorities in fewer districts, whereas Republicans will prevail by narrower margins in a larger number of districts. In 2016 Democrats who beat Republican opponents won an average of 67.4% of the two-party vote in their districts, whereas Republicans who defeated Democrats received an average of 63.8%. This imbalance is partly due to deliberate attempts to create districts that provide such results, and partly just down to the fact that Democrats tend to live more tightly bunched together in cities. Together, these two factors put up quite an obstacle. According to our model, the Democrats need to win 53.5% of all votes cast for the two major parties just to have a 50/50 chance of winning a majority in the House.
If this imbalance were limited to a single chamber of the legislature, or a single election cycle, the Democrats’ frequent carping about a stacked electoral deck might sound like sour grapes. All electoral systems have their oddities. But changes in where Americans live and contradictions in their constitution—a document designed to work with many weak factions that has instead encouraged and entrenched an increasingly polarised two-party system—have opened gaps between what the voters choose and the representation they get in every arm of the federal government. In recent decades these disparities have consistently favoured the Republicans, and there is no reason to think that trend is going to change on its own.There are three underlying issues here. First, the Senate is deliberately set up so that each state has the same number of senators in order to keep the heavily populated states from dominating that chamber in the way that they can the House of Representatives. That aspect of the system is unlikely ever to change, even though a senator from Wyoming (the least populous state with scarcely more than a half million people) represents fewer than 300,000 people whereas a senator from California (the most populous state with almost 40 million people) represents 20 million persons.
Secondly, with respect to the presidential election, we have this strange thing called an electoral college system:
In all the world’s other 58 fully presidential democracies—those in which the president is both head of state and head of government—the winning candidate gets the most votes in the final, or only, round of voting. But due to the “electoral college” system that America’s founders jury-rigged in part to square the needs of democracy with the demography of slavery, this does not hold true for America. States vote in the college in proportion to their combined representation in both houses of Congress. This set-up means that a candidate who wins narrowly in many small and smallish states can beat one who gets more votes overall, but racks most of them up in big majorities in a few big states.This could be changed, at least in theory, as could the third issue--gerrymandering of districts for the House of Representatives (as well as for many state and local issues). There has been a lot of action on this front, although most recently the Supreme Court has punted.