A Very Early Look At The Battle For The House In 2018 Donald Trump is unpopular enough that Republicans could lose the House, but there’s a lot of uncertainty.

Heading into the 2010 and 2014 midterm elections, President Obama was not popular. His job approval rating was at or below 45 percent, and most polls showed more people disapproved of his performance than approved of it. Democrats, in turn, lost a bunch of races in the U.S. Senate and House in both years. They lost, as President Trump might put it, “big league.”

Now, however, with Trump in the White House, the parties have reversed roles and Republicans are somewhat at the mercy of the president’s popularity, as FiveThirtyEight contributor Nathaniel Rakich wrote on Monday. Trump is unpopular, with an approval rating of just 40 percent. If you’re a Democrat thinking about running for office in a swing district in 2018, you’re hoping Trump’s ratings stay on the mildly to very unpopular side of the scale. Actually, “mildly unpopular” might not be good enough. The relationship between presidential approval ratings and congressional elections is rough, and Republicans have some pretty clear structural advantages. The GOP could hold the House even if Trump remains about as unpopular as he is now.

Historically, there’s a wide margin of error between a president’s approval rating and how people vote in House elections. Generally speaking, a president’s party does better in down-ballot races when he’s popular and worse when he’s less popular. But it’s not a 1-to-1 relationship. You can see this by plotting the presidents’ final Gallup job approval ratings before every midterm election since World War II against how many House seats his party lost.

Let’s use Trump’s current approval ratings, 40 percent, to show how inexact this relationship is. If we were able to predict the House results perfectly based on Trump’s 40 percent approval rating, Republicans would be forecast to lose roughly 40 seats. Democrats need to pick up 24 seats to take back the House. But the margin of error on this estimate is +/- 33 seats! That is, Democrats could gain as few as seven seats or as many as 73. Democrats could end up on the upper end of that scale, if nearly every person who dislikes the president votes for the Democratic candidate in their House election. We have, after all, seen an increasingly strong relationship between how people feel about the president and how they vote for Congress.

But Democrats could also just as easily fall short of a majority. They may not do a good job of recruiting top-notch candidates. In 2016, Democrats didn’t even field a candidate in 29 districts, including in Republican Rep. Pete Sessions’s district in Texas, even as Hillary Clinton beat Trump there. There’s also the issue of district lines. The way district lines are currently drawn benefits Republicans by distributing GOP voters more efficiently than Democratic voters. So, all else being equal, we would probably expect Republicans to win more seats than Trump’s approval rating alone indicates.

One way to get around the effect of how the districts are drawn is to try to predict the national House vote — the change in the share of people nationally who vote for Democratic and Republican House candidates — instead of how many House seats change hands. By looking at the national vote, we can get a sense for how many people are voting Democratic or Republican before those votes are skewed by the congressional district lines. But even when looking at the change in the national House vote, there’s a wide margin of error.

If Trump’s approval rating stayed at 40 percent in 2018, Republicans would be expected to lose the national House popular vote by 10 percentage points (the GOP won it by 1 point in 2016). That’s a shift of 11 percentage points from 2016. That size loss would probably be big enough that Republicans would lose control of the House, gerrymandering and urban packing notwithstanding. But that projection comes with a margin of error of about +/-8 percentage points. That could mean an even a larger win for Democrats, which wouldn’t be too surprising given the enthusiasm we’ve seen from Democrats in marches and town hall meetings. Or, a much smaller one.

Recent midterm elections have been tough on Democrats. Compared to years with a presidential election, the electorate in midterm cycles has been older and whiter. Both of those blocs lean Republican. So surveys of all adults — such as Gallup’s — could be underestimating Trump’s popularity with the 2018 midterm electorate (if it looks like recent midterm electorates). Additionally, Congress’s approval rating seems to be up. And while presidential approval ratings are a larger driver of vote choice, congressional approval ratings also matter in midterm elections.

So if Democrats win the national House vote by a margin in the low- to mid-single digits, that may not be enough to take back the House. The median congressional district was 5.5 percentage points more Republican-leaning in the presidential race than the nation as a whole in 2016, meaning Democrats are essentially spotting the GOP 5.5 points in the battle for control of the House. And even that may be underestimating Republicans ability to win a majority of seats without a majority of the vote. Since 2012 (or when most states instituted the current House district lines), Republicans have won, on average, 51 percent of the two-party House vote and 55 percent of House seats. If that difference holds for 2018, Democrats would need to win the House popular vote by about 8 percentage points to win half the House seats.

Of course, we’ve been using Trump’s current approval rating this whole time just to illustrate how much uncertainty there is. But Trump’s approval rating could improve — or fall even further — by the time people vote in November 2018. The average first-term president has seen his approval change by 9 percentage points from this point through his first midterm election. If Trump’s approval rating were 49 percent, Democrats would be slated to pick up 27 seats with the same +/- 33 seats margin of error. If, however, Trump’s approval falls to 31 percent, Democrats would be projected to gain 53 seats (with the same +/-). Put simply, there’s a wide range of possible outcomes.

Trump’s low approval rating is good news for Democrats. But they’ll have to work to capitalize on the national environment, or they might fall victim to the same structural forces that hurt Clinton and fail to take advantage of Trump’s unpopularity.