The 2018 midterms are a story of two chambers. Democrats are in the best position they’ve been in since 2010 to win a majority of seats in the House of Representatives. The Senate map, on the other hand, is so tilted toward the GOP that most political analysts have all but dismissed Democrats’ chances of winning the chamber before 2020. It has even been suggested that Republicans could gain enough Senate seats (eight) in 2018 to amass a filibuster-proof majority (60 seats).

This is normally the part of the article where I push back on the conventional wisdom and argue something like, actually, the 2018 Senate map isn’t that bad for Democrats. But no, it’s pretty bad: Democrats are a long shot to take back the Senate.

What I will argue, however, is that it’ll also be difficult for the GOP to pick up a bunch of seats. Republicans would need to oust incumbent Democrats, and it’s extremely difficult to beat an incumbent senator in a midterm when his or her party doesn’t control the White House.

It may seem a little nuts to suggest that Senate Minority Leader Chuck Schumer can keep losses to a minimum in 2018. Democrats hold 23 of the 33 seats up for a vote. There are 10 Democratic senators running in states that President Trump won, five of whom (Sens. Joe Donnelly of Indiana, Heidi Heitkamp of North Dakota, Joe Manchin of West Virginia, Claire McCaskill of Missouri and Jon Tester of Montana) are from states that Trump won by about 20 percentage points or more. Meanwhile, there are only two Republican senators (Arizona’s Jeff Flake and Nevada’s Dean Heller) up for re-election in states Hillary Clinton came within 5 points of winning in 2016.

But while a lot of Democrats are up for re-election in red states, there’s also a Republican in the White House, and incumbent senators in the opposition party — for simplicity, let’s call them “opposition senators” — tend to survive in those situations.

INCUMBENT SENATORS MIDTERM RAN WON WIN RATE AVG. WIN MARGIN 2014 11 11 100% +29 2010 11 11 100 +30 2006 14 14 100 +30 2002 11 10 91 +24 1998 14 12 86 +26 1994 10 10 100 +27 1990 16 16 100 +27 1986 9 9 100 +29 1982 18 17 94 +27 Total 114 110 96 +28 Opposition party Looking only at senators who ran in the general election. The average margin of victory does not include senators who were beaten in the primary and then ran as a third-party candidate in the general election.

There have been 114 opposition senators who have run in a midterm general election since 1982. Only four of the 114 (4 percent) lost. Most won by wide margins, with the average opposition senator beating the candidate of the president’s party by 28 percentage points. Even in the worst year for opposition senators (1998), 86 percent were re-elected. If 86 percent of incumbent Democrats win in 2018, the party would lose three seats. That would leave Republicans with 55 seats, a more comfortable majority but far short of filibuster-proof.

In contrast, senators in the same party as the president running in midterm years — this will be Republicans in 2018 — lose fairly often.

INCUMBENT SENATORS MIDTERM RAN WON WIN RATE AVG. WIN MARGIN 2014 15 10 67% +7 2010 10 8 80 +16 2006 14 8 57 +15 2002 15 14 93 +31 1998 15 14 93 +24 1994 16 14 88 +14 1990 14 13 93 +24 1986 18 12 67 +13 1982 11 10 91 +7 Total 128 103 80 +17 President’s party Looking only at senators who ran in the general election.

Of the 128 senators who fit this description, 25 (20 percent) lost re-election. In every midterm since 1982, at least one incumbent of the president’s party was defeated. If that trend held this year, it would probably mean that either Flake or Heller would go down. In some years, a third or more of incumbents of the president’s party lost in the general election. Overall, these senators’ average margin of victory has been 17 percentage points, 11 points worse than opposition senators.

Of course, these numbers don’t take into account how blue or red each seat is. Here are the re-election rates for the 42 opposition senators since 1982 who were up for re-election in states that leaned toward the other party. (A Democrat running in a red state, for example.)

INCUMBENT SENATORS MIDTERM RAN WON WIN RATE AVG. WIN MARGIN 2014 1 1 100% +37 2010 1 1 100 +31 2006 4 4 100 +30 2002 5 4 80 +11 1998 3 2 67 +18 1994 4 4 100 +16 1990 10 10 100 +28 1986 5 5 100 +22 1982 9 8 89 +22 Total 42 39 93 +22 Opposition party, state voted for president Looking only at senators who ran in the general election. The previous two presidential election results in a state are use to determine the “lean” of a state compared to the nation as a whole. The previous election is weighted 75 percent and the one before that 25 percent.

Of those 42, 39 (93 percent) won re-election. Most of the time, the race wasn’t even close; the average senator won by 22 percentage points.

Now, look at the re-election rate for the senators in the same party as the president running for re-election in states that lean toward their party. (That is, the situation facing all Republican incumbents except for Dean Heller in 2018.)

INCUMBENT SENATORS MIDTERM RAN WON WIN RATE AVG. WIN MARGIN 2014 10 9 90% +14 2010 9 8 89 +20 2006 11 7 64 +16 2002 14 13 93 +32 1998 10 9 90 +27 1994 10 9 90 +20 1990 11 11 100 +26 1986 11 7 64 +13 1982 6 5 83 +6 Total 92 78 85 +20 President’s party, state voted for president Looking only at senators who ran in the general election. The previous two presidential election results in a state are used to determine the “lean” of a state compared to the nation as a whole. The previous election is weighted 75 percent and the one before that 25 percent.

Their re-election rate (85 percent) in general elections is actually worse than opposition senators running in hostile territory. Their average margin of re-election was also slightly smaller.

So all that should cheer red-state Democrats contemplating their re-election bids in 2018.

If you’re a Republican senator running for re-election in 2018, you’re hoping that it resembles 1998 or 2002, the only midterms since 1982 when a higher share of presidential party incumbents won seats than opposition senators did in states that leaned toward the president’s party. Or, maybe 1990, when those two groups broke even.

Of course, at the moment, it seems unlikely that 2018 will look like those elections. In all those years, the president had an approval rating of 58 percent or better on Election Day, according to Gallup. In fact, three of the four opposition senators to lose since 1982 (Al D’Amato in 1998, Lauch Faircloth in 1998 and Max Cleland in 2002) ran in years when the president’s approval rating was at 63 percent or above. The current political environment looks nothing like 1990, 1998 or 2002. In fact, it’s terrible for Republicans right now. According to the FiveThirtyEight Trump approval tracker, the president’s approval rating right now is just 39 percent.

So does this mean that all the vulnerable Democratic incumbents are going to win? Not really. There are a few reasons to be cautious when looking at the past midterm success of opposition senators in states that lean in favor of the president’s party.

First, the lean of a state at the presidential level has become increasingly predictive of Senate results. That’s especially the case for 2010 and 2014. Yet in those two elections, we only have had two opposition senators (Susan Collins in Maine in 2014 and Chuck Grassley in Iowa in 2010) who ran for re-election in hostile territory. . In both cases, the states leaned less toward the president’s party than Indiana, Missouri, Montana, North Dakota and West Virginia do now. Although both Collins and Grassley easily won re-election, it could make the case that in our current political atmosphere the strong Republican lean in any of the five aforementioned states may be too much for one or more of these incumbents to overcome.

Second, we’re in even more uncharted territory when it comes to Heitkamp in North Dakota and Manchin in West Virginia. Dating back to 1982, there isn’t a single example of an incumbent in the opposition party even running for re-election (let alone winning) in states that leaned as much toward the president’s party as North Dakota and West Virginia (34 and 40 percentage points, respectively, did in the weighted average of the previous two presidential elections). Both states have also trended away from the Democrats, as Trump took them by 15 percentage points more in 2016 than Mitt Romney in 2012.

Third, we cannot be sure that all these red-state Democratic senators are going to run for re-election. Although most have said they will, they could change their minds or lose in a primary. If either of those scenarios happens, keep in mind that the default political lean in a state tends to be much more determinative in open elections in midterm years — red states vote Republican and blue states vote Democratic.

It’s far too early to know how 2018 will play out. It’s a bad map for Democrats, and Republicans are working against history with Trump in the White House. Maybe one of those forces will swamp the other. Or maybe the result will be a wash.