An interesting article in Sabato's Crystal Ball on how the president's party, from Truman through Obama, consistently loses seats in Congress and statehouses speculates about what state politics would look like today if Sen. John McCain had won the 2008 presidential election.
Again, let’s just imagine what would’ve happened if we hold all else equal in 2008, except that McCain wins instead of Obama. Not only do Democrats probably retain their state legislative power (they held total control of 27 states after 2008), but they also would probably have retained influence over redistricting in at least some of these aforementioned states. Again, Obama’s 2008 victory — and his subsequent rebuke in the 2010 midterm — has had long-term repercussions for Democrats. For example, it is possible that the 2010 election essentially ceded control of the U.S. House to the Republicans up through the year 2022, when the next redistricting takes effect — determined in good part by the outcome of the 2020 contests.
The article includes lots of historical political detail and justification for this hypothetical and a nice shout-out for NCSL's Tim Storey, who provided the state legislative data and some of the analysis.