Democratic senators from states Trump narrowly carried last year, like Pennsylvania’s Bob Casey, should have a pretty good wind at their backs in 2018. Photo: Aaron P. Bernstein/Getty Images

As the political terrain for 2018 continues to come into view, most of the buzz is about the U.S. House, and the robust but hardly certain possibility of a Democratic takeover. That is partly because the Senate landscape seems impossibly difficult for the Donkey Party, which is defending 25 of 33 seats up next year, ten of them in states carried by Donald Trump last year. And of the Republican incumbents who are defending their seats, only two appear vulnerable at this point (though both, Jeff Flake and Dean Heller, are now really vulnerable, facing both primary and general-election opposition). Everyone understands that the pro-GOP landscape and the anti–White House midterm dynamics are pushing these races in different directions. But it’s hard to get a handle on the magnitude of these contending forces.

Larry Sabato’s Crystal Ball filled in some of our knowledge gap today, with a study by Geoffrey Skelley of Senate races in midterms dating all the way back to 1914. The numbers are pretty clear: “91% (287 of 314) of non-presidential party incumbents won reelection in midterms.” That’s much better than the 75 percent midterm reelection rate of senators from the presidential party.

Lest you imagine this is all a vestige of a bygone era of wide-scale ticket-splitting, there’s this factoid:

If anything, out-party incumbents losing in a midterm is becoming less common: In six of the last eight midterms, including the last three (2006, 2010, and 2014), no such incumbent lost reelection.

Here’s the fun part of the Crystal Ball analysis:

Applying the historical averages to next year’s Senate elections would result — drumroll please — in a net party change of…zero seats. If 91% of the Democrats/Democratic-caucusing independents are reelected, that would be 23 out of 25, and if 75% of the Republicans are reelected, that would be six of eight, leading to no net change.

Without making any predictions, such a scenario is plausible: Democrats could lose two of the incumbents defending dark red states, in states such as Indiana, Missouri, Montana, North Dakota, and West Virginia, but otherwise hold everything else, while Republicans could lose the only two seemingly vulnerable seats among their much smaller stable of incumbent-held states: Arizona and Nevada.

It is clear that the tendency to lump all of the so-called Trump Ten senators into the same endangered category has never made much sense. Trump carried Florida, Michigan, Pennsylvania, and Wisconsin by narrow enough margins that even the mildest midterm anti–White House breeze could carry Democratic incumbents over the finish line. Ohio (an 8-point Trump margin in 2016) is more promising for Republicans, with Indiana (a 19-point margin) and Missouri (18 points) even more promising, and Montana, North Dakota, and West Virginia (20-point-plus margins) really ripe targets. But Democratic incumbents in all these states have their own distinctive strengths, and at the moment not one of them is trailing according to the authoritative Cook Political Report.

There is one more imponderable to throw into the mix: Will the tendency of demographic groups that have recently become central to the Democratic coalition to significantly underperform in midterms change? There is reason to believe, in part based on special elections since Trump took office, that the spirit of resistance to the 45th president will offset some if not all of that phenomenon. But it’s something to keep in mind.

In general, the shape of the Senate after 2018 is going to be conditioned by a variety of competing and conflicting factors. One thing, however, is very clear: Had Hillary Clinton won last November, we would probably be looking at big Senate — and House — gains for the GOP next year. Using the in-party/out-party historical percentages, here’s what you’d get according to Crystal Ball’s Kyle Kondik:

Just going by the averages, we’d expect the Democrats to lose a half-dozen seats and the GOP to lose one or even zero incumbents. And the net Democratic losses probably would have been even bigger because it’s easy to imagine one or more red state Democrats seeing the writing on the wall and retiring in advance of a tough midterm, giving the Republicans an easy open-seat pickup or two.

In other words, had Clinton won, we might be talking about a GOP drive to get a 60-vote supermajority in the Senate. There are always silver linings for the party that falls short in presidential elections.