Democrats have a path to winning a House majority next year, but that possibility is highly dependent on variables over which they have effectively no control. That’s the takeaway from our initial ratings of 2018’s House races, a list that is heavy on Republicans who start this cycle only mildly endangered.

Historically, the president’s party loses ground in a midterm: That’s what happened in 36 of the 39 midterms since the Civil War. American political history is dotted with elections where the president’s party suffered big losses because of a bad economy (1930, 1938, 1958, and 2010 are all examples), unpopular wars (1950, 1966, and 2006), scandal (1974), or other factors. If a wave is developing, we may be able to track it through President Trump’s approval rating (particularly if it falls into the 30s) and/or the House generic ballot. Watch to see if the latter metric, which measures national opinions on how voters intend to vote in their local House race, starts to show a significant Democratic lead approaching double digits in polling averages. Those are the kinds of numbers it’s probably going to take for Democrats to crack the GOP’s House majority, which is protected both by the power of incumbency and by favorable district lines in many key states. Those structural advantages can help Republicans next year, although they do not grant the party an impregnable firewall, particularly if the economy slows or the public reacts poorly to whatever the White House and Congress ultimately decide to do about health care.

Democrats only begin with a short list of real targets next year. These are the seats that start in the Toss-up and Leans Republican columns, seats held by incumbents who have won tough races but who also mostly started their House careers during Barack Obama’s presidency and thus have not had to run a race with a Republican in the White House. But Democrats have a much longer list of potential targets — the Likely Republican seats, which they could put into play if the national mood breaks in their favor (and if they produce credible challengers, which is something that is in their control).

Last year, voters elected 241 Republicans and 194 Democrats to the House. Since then, vacancies have reduced the House to a 237-193 Republican advantage. Assuming the incumbent party holds all five of the vacancies (more on that below), Democrats would need to net 24 seats to take a House majority.

Our initial ratings include just 62 of the 435 House seats (14% of the total seats) as competitive to at least some degree. Of those, 41 are held by Republicans, and 21 are held by Democrats — which means that the remaining 200 Republican seats and 173 Democratic seats not listed on our ratings seem safe for the incumbent party to start.

Of the 62 seats rated as something less than safe, about half of them (30) are rated in the Likely Republican column. These are seats that are just potentially competitive: In a neutral environment, very few, if any, would be in much danger. Meanwhile, just 11 Republican-held seats are in the more competitive Toss-up and Leans Republican categories. Sweeping these districts wouldn’t even get Democrats halfway to the 24-seat net gain they need. So they are going to need to expand the playing field, which is where those Likely Republican districts come in.

Table 1: Crystal Ball House ratings

Out of an abundance of caution, we have included every “crossover” district in our initial list of ratings. Those are the districts that voted for different parties for president and House last year. There were 35 of these districts: 23 Republicans hold seats that Hillary Clinton won, and 12 Democrats hold seats that Donald Trump captured. These crossover seats make up a majority of the competitive seats on both sides (23 of the 41 Republican-held districts listed and 12 of the 21 Democratic-held ones). We did an in-depth list and analysis of these seats last month, and there’s not much to add at this early point. However, one thing to watch is whether any of these seats become open. For instance, many believe Rep. Tim Walz (D, MN-1) is gearing up to run for governor instead of another term in his southern Minnesota district, which Trump won by 15 points after Barack Obama carried it by a point and a half four years ago. If this seat is open, Republicans probably would have even odds to win it, but with Walz running the Democrats are favored to hold it. Another potential gubernatorial candidate is Rep. Kevin Yoder (R, KS-3), whose suburban Kansas City district voted for Clinton by a point after Mitt Romney had won it by 10 points in 2012. His district would also be a Toss-up if it became open, but he’s the favorite if he stays put.

Overall, retirements are an important metric to watch because they can be more susceptible to takeover by the other party.

Speaking of, and as noted above, there are five vacant House seats right now: Democrats should easily hold the Los Angeles-area CA-34, and Republicans should easily hold KS-4 and SC-5. Montana’s open at-large district is hypothetically competitive — Democrats can compete in the Republican-leaning state — although Democrats appear more focused on the other special election in GA-6, a suburban Atlanta seat that voted Republican for president by 20 points or more in recent years until Trump only won it by a point and a half last year. We rate the MT-AL and GA-6 specials as Likely Republican and the others as safe for the incumbent party.

Both the National Republican Congressional Committee and the Democratic Congressional Campaign Committee have released their initial target lists, and many of the districts they include do not make the cut as being competitive on our ratings. For Republicans, we’re skeptical that they can truly make inroads against some Democratic incumbents, such as Reps. Scott Peters (D, CA-52) and Raul Ruiz (D, CA-36), whose districts voted more Democratic for president in 2016 than 2012. Meanwhile, Democrats argue that with the right candidates they can put several Trump-friendly heartland districts into play, such as those held by Reps. Andy Barr (R, KY-6), Mike Bishop (R, MI-8), Bob Gibbs (R, OH-7), and Alex Mooney (R, WV-2). That seems like a significant stretch, but perhaps one or more of these districts makes it onto our competitive list depending on the emergence of strong candidates and the national environment. For what it’s worth, Democrats say that they are seeing considerable interest from candidates in a wide range of districts who are inspired to run by the Trump presidency. Whether those candidates actually run, and whether they can perform in red districts, is an open question, although Democrats did have success in some GOP-leaning districts in their 2006 midterm victory.

Conclusion

While the overall environment will dictate a lot of what happens next year, developments in individual districts and states matter, too. For instance, Texas might have to draw a new congressional map for next year: A federal court ruled late last week that the Republican-controlled Texas state legislature had improperly diluted the voting power of minorities when they drew the state’s congressional map after the 2010 census. If the map is eventually redrawn, Rep. Will Hurd (R, TX-23), a narrow winner in both 2014 and 2016, could find his district even harder to defend, and a safe seat or two could also be imperiled, like Rep. Blake Farenthold’s (R) TX-27. Court-ordered, mid-decade redistricting helped Democrats in Florida and Virginia last cycle; in fact, one could argue that redistricting was responsible for half of the party’s modest six-seat net gain last year. But it’s premature to factor a Texas remap into these ratings, which at the moment reflect the current districts.

Watch for retirements or for members who decide to run for other offices, particularly in districts that are already rated as something less than safe for the incumbent party. But also remember that filing deadlines for primaries are a year or more away in many places, and even after those pass there can be mechanisms to replace already-nominated candidates on the ballot. If things go south for either party a year from now, it’s possible that some endangered members could just decide to pass on running again.

Overall, though, voters’ perceptions of Trump and congressional Republicans will loom large next year — or at least history suggests those factors will be important. If perceptions are neutral or broadly positive, the GOP should have little trouble keeping the House. If they are negative, the House will be in play, and some of those Likely Republican districts — the districts that truly will make or break the GOP House majority — might start to slip away.