For over six months this column has suggested that this election amounts to a Democratic tidal wave crashing against a Republican seawall—the question is which will be stronger. At Labor Day, the traditional beginning of the general-election-campaign season, that continues to be the case, though how that dynamic manifests itself will likely vary between the House and Senate, governorships, and state legislatures.

There is no mistaking this wave. For better or worse, midterm elections are referenda on the president. With roughly one-third fewer people voting in midterms than in presidential years, who votes and who stays home is critically important. People who are angry or fearful tend to be disproportionately more motivated to vote in midterm elections than those who are satisfied or complacent. President Trump’s approval ratings have typically been the lowest of any post-World War II elected president in their first two years. Only occasionally have his numbers ticked above those of Jimmy Carter’s in 1977 and 1978. They have trailed those of Presidents Clinton and Obama, both of whom suffered catastrophic losses in their first midterm election, and those of President Reagan, who had moderate 1982 losses.

Both Presidents Bush had strong poll numbers at this point—the elder Bush only lost a small number of House seats in 1990, during the Gulf War, while the younger Bush actually gained seats in 2002, after the 9/11 attacks. The key variable between a bad night for the GOP and a horrible night is whether those Republicans who are not part of either the tea party or white evangelicals—Republicans who are not enthusiastic Trump backers—vote on Election Day.

Unforeseen events could occur, but history shows that between mid-summer and November, midterm waves remain constant or grow in size, and have not reversed direction or diminished. Still, this is where the GOP’s wall comes into play. In the House, the wall is comprised of congressional-district boundaries, drawn in many states by Republican legislatures and governors at the beginning of this decade. Many state legislative boundaries were drawn to the GOP’s advantage then as well. There are exceptions of course, like Illinois and Maryland, where Democrats had the gerrymander pens in their hands, but those are rare.

Then there are natural population patterns, with Democratic voters highly concentrated in urban areas and Republican voters more efficiently allocated outside of the cities, working to protect many GOP majorities in the event of a mild to moderate-sized Democratic wave. We are most likely looking at a Democratic gain in the House of between 20 and 40 seats—with the odds of going higher than 30 and maybe 40 are greater than going lower than 30 or 20 seats.

While the House, with all of its seats up every two years, has long been a highly sensitive political barometer of the national mood, the Senate is less so. With just one-third of its seats up every two years the upper chamber behaves in a more idiosyncratic way. What happened six and 12 years earlier is crucial; one time a group of Senate seats are up in a presidential-election year, the next time that group is up in a midterm-election year. As a result, the rhythms of Senate elections are totally different from the House, or for that matter, gubernatorial and state legislative races.

This 2018 group of Senate seats was last up in 2006 and 2012, two very good years for Democrats, one reason why this is the most lopsided Senate map in modern history, one very favorable to Republicans. The GOP has just nine seats up, only one in a state (Nevada) won in 2016 by Hillary Clinton, while Democrats have 26 seats up, 10 in states where Trump prevailed. The Republican wall defending their Senate majority appears to be substantially higher than in the House; only with the biggest of waves do Democrats get the gain of two seats or more to win a majority. If Democrats had a fantastic year, they could score a net gain of two to four seats, but if the map trumps the wave, you could easily see Democratic losses in the two-to-four-seat range. Between Democrats gaining four seats to Republicans gaining four seats, there are nine potential net-change outcomes, and in six out of nine, Republicans maintain their majority. The kicker is that with so many small states in play, there could be a relatively small number of votes making the difference in a handful of races.

With three-quarters of the nation’s governorships and four-fifths of the state legislative seats up this year, almost all with four-year terms and last up in the pro-GOP 2010 and 2014 Obama midterm elections, Republicans now have huge exposure. In many states, like in the House, those district boundaries serve as protective walls against Republican losses. Tim Storey, the elections guru at the National Conference of State Legislatures, points out that from 1902 through 2014, the average midterm election outcome was a loss of 412 seats (including the previous odd-year’s elections); since World War II the losses have been a bit lower, 334 seats. Storey estimates that with a generic-congressional-ballot-test advantage of Democrats up by 6 points, that would likely translate into a gain of close to 500 state legislative seats nationwide for Democrats. Like in the U.S. House, the curve is asymmetric, the chances of over 500 are greater than under 400.

Republican governors do not have the favorable red-blue map that the GOP senators have, or the district lines advantage that Republicans in the House and state legislatures have in many states. So Republican prospects could be particularly ugly, with a net loss of five or more governorships.

This story was originally published on nationaljournal.com on September 5, 2018