The Structural Background of the 2018 Midterms
By 
Nate Cohn observes that while the Democratic lead on the generic ballot is predictably narrowing, the margin Dems would need on the generic ballot to take the House is also narrowing:
The Democratic advantage on the generic congressional ballot has slipped over the last few weeks. But Republicans have gradually lost advantages of their own.
Slowly but surely, the considerable structural advantages — like incumbency, geography and gerrymandering — that give the Republicans a chance to survive a so-called wave election are fading, giving Democrats a clearer path to a House majority in November.
The Republicans still retain formidable advantages, enough to win the House while losing the national popular vote by a wide margin. But their edge has shrunk considerably over the last few months, and even more over the last few years.
[…]
Over all, the number of G.O.P. retirements in plausibly competitive districts isn’t extraordinarily high. But some of the Republican retirements have been especially damaging: longtime incumbents who have a tradition of running far ahead of the national party and dissuading strong challengers, like New Jersey’s Frank LoBiondo or Pennsylvania’s Charlie Dent. Their retirements could easily be the difference between a non-competitive race and a Democratic victory.
The Republican incumbency advantage has diminished in another way: Democratic recruitment and fund-raising. A strong Democratic recruit — like a military veteran or an elected official — can cut into that advantage, especially with strong fund-raising numbers.
Definitely worth reading the whole thing.
The outrage that Dems could take the House vote by 5 or 6 points and still not regain control shouldn’t be normalized. But the Dems have a very reasonable chance of overcoming the massive structural disadvantages and winning anyway.