One of the arguments going around the web is that Democrats would've won the House if not for Republican gerrymandering. I addressed this in January. Democrats would've likely gained around 8-9 seats. That's something but not enough to win a majority. The reason is Democratic clustering in urban areas. A good way to show this is to show the difference in Republican and Democratic winning margins in similar districts.
The first line in the chart below shows the total votes for each party in the 50 districts congressional Republicans won by the largest margins and Democrats won by the largest margins. Democrats won their 50 districts by 1,321,153 more votes than Republicans won their districts. Overall, Democrats had 1,365,441 more votes than Republicans. So it's almost all from the 50 most Republican and 50 most Democratic congressional districts. What makes this more remarkable is that 24 of the Republican districts had no Democrat running, while only 13 of the Democratic districts had no Republican running. Of the 100 districts, 87 had Republicans, while only 76 had Democrats.
The next 100 districts produces similar results, with Democrats having a 1,343,245 vote edge. While a Democrat ran in all 100 of these districts, there were only 95 Republicans. That's because there are many Democratic dominated districts where turn-out is low. That means that Democrats are at a disadvantage since they can have a smaller vote margin of victory in districts where they get a higher percentage of the vote than a Republican does in a district he wins.
You have to go down to the last 50 districts Democrats won and districts 151-200 that Republicans won for you to get to a better GOP margin. Republicans also won 33 additional district. [Note: There are only 433 districts accounted for because Florida doesn't count votes in unopposed districts. There were one Democratic and one Republican district that didn't have votes counted.]
There are some districts gerrymandered by Republicans in the Democrats top 50 and top 100 victory margins, but this also districts in New York and California, which were independently drawn, and districts in Massachusetts, Maryland and Illinois, which were drawn by Democrats. You know Democratic voters are highly clustered if they overflow even in districts Democrats draw.
If Democrats want a majority in the House they'll either have to find a way to appeal to non-urban voters better or get their voters to move.