Bloomberg has an excellent series on the GOP’s lock on the House of Representatives.
“One big reason the Republicans have this edge: their district boundaries are drawn so carefully that the only votes that often matter come from fellow Republicans.”
“The 2010 elections, in which Republicans won the House majority and gained more than 700 state legislative seats across the nation, gave the party the upper-hand in the process of
redistricting, the once-a-decade redrawing of congressional seats. The
advantage helped them design safer partisan districts and maintain their
House majority in 2012 — even as they lost the presidential race by
about 5 million votes.”