So as we all prepare to watch the returns Tuesday, here are a few conceivable ways it could go.
1. Democrats win the House, while the GOP holds the Senate and even gains a seat or two
How it would happen: This appears the most likely scenario, if you’re to believe the polls.
Republicans have a 51-49 majority in the Senate, which means Democrats need a net gain of two seats to take the chamber. That’s unlikely, by all accounts, because the map is so friendly for the GOP. Despite a favorable environment for Democrats, Republicans have hung tough when it comes to possibly nicking a Senate seat or two (or three) in the many red states Democrats have to defend — possibly offsetting GOP losses in Arizona and Nevada and maybe even leading to a net gain for Republicans.
In the House, Democrats need a net gain of 23 seats. They are favored to win 18 GOP seats, according to the Cook Political Report, and Republicans are favored to win two Democratic seats. That means Democrats would only need to win seven of the 30 seats that Cook rates as “toss-ups.” This would seem likely even in a neutral environment, and history shows that these “toss-up” races tend to go strongly in the direction of the party with the electoral momentum.
What it would mean: If Democrats' House gain is somewhere in the neighborhood of 30 seats, that would be on a par with what often happens in midterm elections when the other party has the presidency. But it would be remarkable when you consider this: The GOP’s redistricting advantage after the 2010 election was so huge that we wondered whether it was even possible that Democrats could win back the House. The median House district went for Trump by more than three points, even though he lost the popular vote by two points. If the map were anywhere close to neutral, and the environment were as it is, Democrats would be virtually guaranteed a House takeover right now. The fact that they are likely to win back a chamber that seemed so out of reach for the entire decade is big, in and of itself.
Republicans — and President Trump — would counter this by pointing out that they held the Senate or even gained seats (which is rare). But there, the map was even tougher, with Democrats defending 10 Trump states and Republicans defending only one won by Hillary Clinton. And a status quo election would actually set up Democrats very well to take over the chamber when they have much better opportunities in 2020 and 2022.
2. A blue wave consumes the House, and the Senate is tight
How it would happen: Let’s assume we’re underselling the size of the blue wave. Let’s assume the double-digit leads that Democrats used to have on the generic ballot (until recently) prevail on Election Day, and Americans vote Democratic by 10 or more points nationwide.
In that scenario, we’d be talking about the vast majority of those 30 toss-ups going to the Democrats, along with potentially some upsets in districts Republicans are expected to hold. The gains would be north of 40 or 45 seats and give Democrats a substantial House majority — comparable to the GOP’s current 42-seat majority.
That environment still might not be good enough to deliver the Senate, though. Republicans would probably lose Arizona and Nevada, the two swing seats they are defending, but they are clear favorites to unseat Sen. Heidi Heitkamp (D) in North Dakota. That means the wave would have to deliver Democrats one more seat — a tough pickup in either Tennessee or Texas — and then help them hold everything else: Florida, Indiana, Missouri, Montana and West Virginia. All of those states are polling competitively, but it’s not inconceivable that they’d hold all four (Republicans hold a tiny lead in the polling average in only one of the four — Missouri).
The biggest question in this scenario is whether Democrats would actually be able to get Beto O’Rourke across the finish line in Texas (where Democrats haven’t won statewide in any race in a quarter-century) or pull an upset for the Tennessee open seat between Rep. Marsha Blackburn (R) and former governor Phil Bredesen (D), where the polling average has Blackburn up by five.
What it would mean: This would be a clear repudiation of Trump, and there would be no other way to spin it. We can argue about the definition of “wave,” but 40-45 seats certainly qualifies, and Democrats would also likely have at least a 50-50 tie in the Senate (though Vice President Pence would still give the GOP control).
3. Democrats somehow win both chambers, with the blue wave also consuming the Senate
How it would happen: This would be the really big wave — and it’s not inconceivable. Projections gave Trump around a 1-in-6 shot at winning the presidency in 2016, and those are the odds FiveThirtyEight gives Democrats to win the Senate.
The likeliest outcome would seem to be Tennessee goes Democratic, and Texas possibly following it, giving Democrats a 51-49 or 52-48 majority in the Senate. The best possible outcome would include Democrats also holding North Dakota — and batting 1.000 on defense — and getting as much as a 53-47 Senate majority.
What it would mean: Trump and the GOP losing both the House and the Senate would be a seismic shift, given the inherent advantage Republicans have on both maps. Immediately, talk would turn to how the 2016 election was perhaps a fluke and to how Trump’s divisiveness had backfired. And it wouldn’t be wrong.
4. Republicans save the House and gain ground in the Senate
How it would happen: Speaking of small but substantial possibilities, that’s what Republicans have when it comes to holding the House. FiveThirtyEight has them at a 1-in-8 shot.
This would require the House GOP holding the vast majority of those toss-up races, possibly because of rising enthusiasm from Trump’s fear-infused, immigration-focused base strategy. And if those toss-up races tilted Republican in the House, you’d think they’d also tilt Republican in the Senate, where the terrain is even redder. It could result in states such as Nevada or even Arizona staying under GOP control and deliver states like Florida, Indiana, Missouri and Montana. Even if Republicans can just win red states, that could be a four- or five-seat gain.
The result would be the GOP maintaining control of all the levers of power in Washington and having more room for error in the Senate, with a 53-47, 54-46 or even 55-45 majority.
What it would mean: Pundit chaos. It would suggest — again — that polls might be underselling Trump’s actual base of support and that we continue to fundamentally misunderstand what he’s doing to our political coalitions. It wouldn’t necessarily be a resounding affirmation of Trump — these are only red states the GOP would be winning — but it would lead to some real, justified soul-searching.
5. A wild card
How it would happen: The idea that we might continue to fundamentally misunderstand the country’s political zeitgeist — combined with the 2016 election — mean there could be something completely unexpected. Maybe the GOP narrowly holds the House, but Democrats keep the Senate close. Maybe Democrats win the House, but Republicans pick up a few Senate seats. Maybe Democrats pull a shocker in Tennessee and/or Texas but lose supposedly easier states.
There are simply so many toss-ups — 30 in the House and nine in the Senate — that weird things can happen. It’s really difficult to rule anything out, given we’re in uncharted territory here, and that all of our previous assumptions about how these things work have been called into question. The safest bet is not to assume everything happens as neatly as you expect they will.
What it would mean: The same as above — chaos — except less specifically partisan. The arguments about exactly what the election meant would be all over the place, and it would be tough to say anything with certainty, because we’d be entering a bona fide new political reality.