As I wrote earlier this week, Democrats almost certainly need a systematic polling error to win the Senate. By that I mean: They need for the polls to be off everywhere, or at least in certain key clusters of states, to win the Senate. A polling error in just one or two races (say, Beto O’Rourke wins in Texas) probably wouldn’t be enough: Democrats are defending too much territory and have too many problems elsewhere on the map just to get lucky.

That conclusion about the Senate ought to be fairly intuitive, I think. Even if you credit Democrats with wins in all the toss-up races, that wouldn’t be enough — it would only get them to 50 seats. What might be more surprising is that the same conclusion holds for Republicans in the House. They need for there to be a systematic polling error too. If the polls are about right overall but Republicans are hoping to getting lucky in the swing districts, it probably won’t happen — the odds are stacked heavily against them.

The reason it’s counterintuitive is because you can’t really identify 23 districts that are safe bets to flip to Democrats (that’s the number they need to take the House). In the Deluxe version of our model (the one I’ll be focusing on here), only 193 seats are considered to be “solid Democratic” (at least a 95 percent chance of a Democratic victory). If Democrats won only those seats and no others, they’d actually lose two seats from the 195 they control now. Another 15 seats are “likely Democratic,” where Democrats have at least a 75 chance of winning. Win those as well, and Democrats are … still up to a net gain of only 13 seats.

The model then has 34 seats in its three most competitive categories: “lean Democratic” (eight seats), “toss-up” (16 seats) and “lean Republican” (10 seats). If Republicans win 24 of those 34 seats — assuming everything else goes to form — they’ll keep the House.

The divide between the House outlook and the Senate outlook continues to widen. Democrats’ chances of winning a majority remain at or near their all-time highs in our House forecast — ranging between 78 percent (7 in 9) and 85 percent (6 in 7) in the various versions of our model. But they’re at their lowest point yet in the Senate. All three versions of our forecast give them only about 1 in 7 shot (about 15 percent) of taking over the Senate from Republicans.

This is normally the point at which you might expect us to give you a throat-clearing “well, actually” about how 1 in 7 chances happen all the time. Indeed, they do. One in seven days of the week is a Thursday. None of us woke up this morning screaming “Oh my gosh, I just can’t believe it’s a Thursday!” And nobody should really be that surprised if Democrats win the Senate next week, or if Republicans keep the House.

At the same time, Republicans have a fairly clear advantage in the Senate (as Democrats do in the House) — clearer than the edge Hillary Clinton had before the 2016 election, when President Trump had roughly a 3 in 10 chance to win the Electoral College. In 2016, a normal-sized polling error (if it worked in Trump’s favor) was probably going to be enough to give him a victory in the Electoral College. And that’s exactly what happened: The polls weren’t great in 2016, but they were about as accurate as they have been on average since 1972. Because the race was close and because Clinton was underperforming in the Electoral College, a small and routine but systematic polling error was enough to give Trump the win.

The difference this year is that a normal-sized polling error in Democrats’ direction would merely make the race for the Senate close. (Likewise, a normal-sized polling error in the GOP direction would make the House close, but Republicans would still have to fight it out on a district-by-district basis.) A sports analogy, for those so inclined: In 2016, Trump was doing the equivalent of driving for the game-winning touchdown with the odds somewhat but not overwhelmingly against him. If enough undecided voters in the Midwest broke toward him, he was going to win the Electoral College. In the Senate this year, by contrast, it’s more like Democrats are driving for the game-tying touchdown; they still have to win in overtime even if they score.