Bottom not falling out for House R's. NBC/WSJ poll congressional ballot test 46R/46D; at this point in '08 52D/33R.— amy walter (@amyewalter) June 27, 2016
It's true -- in poll after poll, when you ask voters which party they favor for their House seat, naming no candidate names, the parties are running even, or the Democrats are only slightly ahead, in poll after poll.
If you're assuming that distaste for Donald Trump will translate into distaste for the Republican Party in general, please remember that the news is full of stories about mainstream Republicans' squeamishness about Trump, and about Trump's contempt for the party establishment. The dominant narrative is that Trump isn't really representative of the party. A lot of people read this as a party in chaos, but I think Reince Priebus, Mitch McConnell, Paul Ryan, and other pary leaders are doing a pretty good of generating that Trump-vs.-GOP narrative. They're assuming that voters who like Trump will vote GOP downballot because he's at the top of the ticket, while many voters who don't like Trump will still vote GOP downballot because the pre-Trump party clearly still exists, and regularly expresses contempt for Trump. In 2008, when Democrats led the "generic ballot" question by 19 points in June, the GOP was the party of the despised George W. Bush -- there was no ambiguity about that. But this year it's not clear who really represents the GOP. Would-be Republican voters can believe it's Trump or believe it's the old guard -- whatever they choose.
I'm assuming you know why a 50-50 House vote wouldn't produce a 50-50 House: Redistricting and gerrymandering by Republican state governments means that Democrats have to win the overall vote in a near-landslide just to reach parity in the House. A Republican scheme called REDMAP, initiated in 2010, made sure of that, as David Daley explains in a new book titled Ratf**ked: The True Story Behind the Secret Plan to Steal America's Democracy. In a New Yorker review of the book, Elizabeth Colbert explains how it works, using Pennsylvania as an example:
Two of the most common gerrymandering techniques are “packing” and “cracking.” In the first, the party in charge of redistricting tries to “pack” voters from the rival party into as few districts as possible, to minimize the number of seats the opposition is likely to win. In the second, blocs of opposition voters are parcelled out among several districts, to achieve the same goal.Kolbert adds:
Both techniques were brought to bear in Pennsylvania. The new Republican majority “packed” blue-leaning voters into a handful of districts around Philadelphia and Pittsburgh. Then it “cracked” the rest into districts that tilted red.
... So skillfully were the lines drawn that in 2012 -- when President Obama carried Pennsylvania by three hundred thousand votes and the state’s Democratic congressional candidates collectively outpolled their G.O.P. rivals by nearly a hundred thousand votes -- Republicans still won thirteen of Pennsylvania’s eighteen seats in the U.S. House of Representatives.
In House races in 2012, 1.7 million more votes were cast for Democrats than for Republicans. And still, thanks to the way those votes were packed and cracked, Republicans came away with thirty-three more congressional seats.So how big would the Democrats' overall lead in the House vote have to be if they want to win back the House? At Salon, Daley provides an estimate:
That ... edge in 2012 amounted to 50.4 percent of the two-party vote overall for the Democrats. You would need something upwards of 55 percent to get in the ballpark of the chambers switching, which would require many millions more votes.In other words, a 10-point margin.
And we're nowhere near that, judging from the poll averages, because many people who loathe Trump still don't associate him with their nice GOP House member. And every time an outrageous Trump statement is denounced by an establishment Republican, the House majority gets safer -- and probably the Senate majority as well.