One thing unsaid in the article is the part of the core Dem appeal. It has shifted over the last 50 years from a labor-based appeal to an urban-based appeal. That contributes to the over-concentration problem since their historical labor votes could be found in every area, but small towns and rural areas aren't going to have a lot of pro-urban voters.
Interestingly the IL Dems have maintained a more labor-oriented message and still hold many local offices downstate, even though the Congressional seats go Pub. That has meant a lot of political work to balance competing interests between Chicago and the rest of the state, but that balance has paid off with continued supermajorities in the legislature despite the 2014 results nationwide.
The Illinois senate and assembly is heavily gerrymandered in favor of Democrats.
https://en.wikipedia.org/wiki/Illinois_House_of_Representatives_elections,_2014
Despite a 1% margin for the GOP. The dems got 60% of seats.
I'm sure this is all news to muon
