In a fascinating analysis in The Cook Political Report, Richard E. Cohen explores which areas of the country have essentially become locked-up by either the Democrats or the Republicans, and which areas remain fluid enough to provide the marginal forces that determine national political outcomes. For the House, he concludes that six states essentially determine which party controls that chamber: Ohio, Michigan, Indiana, Florida, Pennsylvania, and Virginia.
Then he goes on to offer the following perspective on the distribution of control over these seats from 2008-today:
Following the 2008 election, those six states had 51 Democrats and 46 Republicans in the Democratic-controlled House. The huge turnover in the 2010 election shifted 21 of those seats to the Republicans, with a five-seat GOP gain in both Ohio and Pennsylvania and at least a two-seat gain in each of the other four states. Strikingly, that 67- to 30-seat Republican control in those states barely shifted following the 2012 redistricting and the three subsequent elections. The 2016 results yielded a 64- to 31-seat advantage for Republicans. (The states have lost two House seats as a result of reapportionment shifts between the states.)
In addition, he notes:
Of those 31 Democratic-held House seats, each is in a large metropolitan area— either in a dominant city or close-in suburbs. Republicans control all of the exurban and rural districts in the six states.