The constitution was not designed for the two-party politics it unwittingly encouraged
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.
With regards to the presidential election:
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.
During almost all of the 20th century this did not matter much; the candidate who got the most votes won every election from 1896 to 1996. But both of the past two Republicans to win the presidency have received fewer votes when first elected than their Democratic opponents did. In the contest between Al Gore and George W. Bush in 2000, this margin was a modest 0.5 percentage points. In 2016, however, it was substantial: Hillary Clinton’s lead of 2.1 percentage points was larger than those enjoyed by the victorious John F. Kennedy in 1960, Richard Nixon in 1968 and Jimmy Carter in 1976.
Source: America’s electoral system gives the Republicans advantages over Democrats – The minority majority