Wednesday, September 5, 2018

538 reports on Dem chances of taking House

538’s significant digits email has the short take on the GOP’s chances of keeping the House. We then consider the Dem’s chances of taking it back.

1 in 5 chance of holding onto the House
Labor Day is behind us. Time to put the white wardrobe into storage, head back to school, and start paying close attention to the midterm elections. According to FiveThirtyEight’s Classic model, Republican congressmen headed back to work this week facing their worst chances to date of holding onto the House. Those chances are 1 in 5, or 19.9 percent, as I write this. [FiveThirtyEight]

The longer version is in this Election Update from Nate Silvers: Democrats Are In Their Best Position Yet To Retake The House.

Labor Day is the traditional inflection point in the midterm campaign — the point when the election becomes something that’s happening right now — then Democrats should feel pretty good about where they stand in their quest to win the U.S. House.

Although it can be a noisy indicator, the generic congressional ballot is showing Democrats in their best position since last winter, with a handful of high-quality polls (including one from our ABC News colleagues) giving them a double-digit advantage over Republicans. Meanwhile, President Trump’s approval rating — as of late Tuesday morning, an average of 40.1 percent of adults approved of his performance according to our calculation, while 54.1 percent disapproved of him — is the worst that it’s been since February.

As a result, Republicans are in their worst position to date in our U.S. House forecast: The Classic version of our model gives them only a 1 in 5 chance of holding onto the House. Other versions of our model are slightly more optimistic for the GOP: The Deluxe version, which folds in expert ratings on a seat-by-seat basis, puts their chances at 1 in 4, while the Lite version, which uses district-level and generic ballot polls alone to make its forecasts, has them at a 3 in 10 chance. Whichever flavor of the forecast you prefer, the House is a long way from a foregone conclusion — but also a long way from being a “toss-up.”

There are three questions that we ought to ask about this data. First, why have the changes in presidential approval and the generic ballot happened? Second, how likely are they to stick? And third, how much do they matter?

Question 1: Why has Republicans’ position apparently been worsening?
Well, I don’t know. The changes are modest enough that they could be statistical noise. … But, the fact that both Trump’s approval rating and the generic ballot are moving in the same direction should make us more confident that the trend is real and should suggest that both indicators have something to do with the president.

Question 2: How likely are these changes to stick?
… The method of calculating the generic ballot that we use on our generic ballot interactive, which currently shows Democrats ahead by 10.8 percentage points, is too aggressive and will usually overestimate swings.4 Our House forecast actually uses a different, slower-moving version of the generic ballot average. In that version, Democrats currently lead the generic ballot by 8.7 percentage points. That’s still pretty good, but the House forecast will need to see Democrats sustain their most recent numbers for a few weeks before it concedes that they’re really up double digits. … Put another way, a slight Democratic uptick on the generic ballot to 8 or 9 percentage points6 is arguably bringing the race for Congress more in line with historical norms. That’s one reason to think it could hold. But we’ll need to see more evidence — not only from the generic ballot but also from other indicators — to conclude that Democrats are really ahead by 10 or 12 or 14 points, which would produce a gargantuan wave.

Question 3: How much does all of this matter?
It might seem like we’re parsing awfully fine distinctions — e.g., between an 8-point popular vote margin and a 7-point one. But they matter, because it doesn’t take that much for Democrats to go from House underdogs to potentially taking 40 or more seats.

Here, for example, is the output from a Tuesday morning run of our Classic forecast, showing how a projected margin in the House popular vote translates into potential seat gains for Democrats. If Democrats win the popular vote by “only” 5 to 6 percentage points — still a pretty comfortable margin, but not necessarily enough to make up Republicans’ advantages due to gerrymandering, incumbency and the clustering of Democratic voters in urban districts — they’re only about even-money to win the House. If they win it by 9 to 10 points, by contrast, they’re all but certain to win the House and in fact project to gain about 40 seats!

Democrats have no room for error
How the popular vote translates into House seats for Democrats per FiveThirtyEight’s Classic model as of Sept. 4, 2018.
From left to right, read
14–15 +66 over 99%
13–14 +61 over 99%
12–13 +56 over 99%
11–12 +51 over 99%
10–11 +46 over 99%
9–10 +41 99%
8–9 +36 98%
7–8 +32 92%
6–7 +27 78%
5–6 +24 56%
4–5 +20 29%
3–4 +16 11%
2–3 +13 3%
1–2 +10 1%
0–1 +7 less than 1%

Silvers goes on to list reasons for Democratic worry: “You mean, aside from the fact that 1 in 5 chances still happen 1 in 5 times, which is kind of a lot?! Or that the Senate map is still very difficult for Democrats, even if the House looks reasonably favorable to them?”

Dems listen up! Silvers’ bottom line is this: “Democrats are well-positioned to win the House — but they still have a lot of work to do to turn good prospects into a reality on the ground.”

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