Democrats got the best of Republicans during the 2017 election season, outperforming expectations in special elections across the country and easily winning the only two governors races. Much of that success came down to turnout — the Democratic base came out and voted.

It’s now 2018, though. A midterm year. And if you’ve followed politics recently, you’re used to hearing what may seem like an immutable law of elections: Midterm turnout favors the GOP. That was incredibly true in the last two midterm cycles, 2010 and 2014. And it’s true generally too. But it masks an important factor: which party holds the White House.

Take a look at the table below. It shows — for every midterm since 1978 — the difference between Democrats and Republicans in self-identified party identification among all registered voters compared with those who voted in the midterms (i.e. the turnout margin or advantage).

A closer look at the Republican midterm turnout advantage The shift toward the GOP in party identification margin from all registered voters to those who voted in the midterm election Republican midterm turnout advantage Under a Dem. president Under a GOP president 1978 (Carter) +6 — 1982 (Reagan) — +1 1986 (Reagan) — None 1990 (H.W. Bush) — +3 1994 (Clinton) +6 — 1998 (Clinton) +3 — 2002 (W. Bush) — +2 2006 (W. Bush) — None 2010 (Obama) +6 — 2014 (Obama) +5 — Average +5 +1 Median +6 +1 Turnout is self-reported for 1982, 1994, 1998 and 2002. Turnout is from verified voter files for all other years. Source: American National Election Studies, cooperative congressional election study

Overall, in the 10 midterm elections since 1978, the average Republican turnout advantage has been about 3 percentage points. In other words, the GOP does about 3 points better, on average, among midterm voters compared with whatever their margin is vs. Democrats among all registered voters. In short, Republicans have a midterm turnout advantage.

There’s a second important force at work during midterm elections, however. The Republican turnout advantage is either exacerbated or all but canceled out depending on which party controls the White House. Scholars have long shown a major loss of support for the president’s party in midterm elections. With a Democrat in the White House, the GOP turnout advantage gets even bigger. With a Republican in the Oval Office, the GOP on average barely has any advantage at all.

Notice how the Democratic wipeouts in 2010 and 2014 fit a long-standing pattern. The Republican turnout advantage those years — 6 percentage points in 2010 and 5 points in 2014 — are both within 1 percentage point of the average for midterms with a Democratic president. Those advantages look a lot like the GOP edge in the 1994 midterm, for example, when Democrat Bill Clinton was president and Democrats lost the House.

Note, however, how different 2006 was. In that midterm, the last one with a Republican president, there was practically no difference between Democrats’ party-identification advantage over Republicans among all registered voters and among those who cast a ballot. We saw roughly the same thing in the average midterm with a Republican in the White House.

Now, to be clear, it’s not as if Democrats should expect a turnout advantage in 2018. Even in Democrats’ best years, the party-identification edge they enjoyed among those who turn out to vote has merely equaled their advantage among registered voters. It’s just that Democrats shouldn’t be harmed too much by a lack of turnout from Democratic voters. If 2018 turnout patterns do, in fact, look like most prior midterms with a Republican president, it would mean that Republicans can’t rely on turnout to compensate for a lack of support.

We should get an early sign of how turnout is shaping up when pollsters begin to show results among likely voters, rather than registered voters. Democrats have about a 10 percentage point lead in the aggregate generic ballot, which mostly consists of surveys conducted among registered voters. If normal turnout patterns hold, that lead shouldn’t shrink that much when pollsters make the likely voter switch. Additionally, pollsters would be making a big mistake if they try to weight their samples by who turned out in the last midterm. We already saw in Alabama how weighting to the 2014 electorate produced numbers too favorable to Republicans.

Of course, there is no guarantee that 2018 turnout will look like 2006 or other midterms with a Republican president. President Trump has defied history before. So it’s possible that come Election Day 2018, Republicans will have a clear turnout advantage.

That happened, to a certain extent, in 1990 and 2002, when the GOP’s margin in party identification among midterm voters was clearly better than it was among all registered voters, though the turnout advantage was still lower than the average when a Democrat has been president. Now, the national political environments in 1990 and 2002 looked very different than the environment today. In those years, the presidents had approval ratings of 55 percent or higher. So far this year, Trump’s approval rating is south of 40 percent, and many more strongly disapprove than strongly approve of the job he is doing as president. That would suggest that Democrats should be highly motivated and that 2018 turnout is more likely to follow the script of 1982, 1986 and 2006 than it is to follow 1990 or 2002.

The 1990 and 2002 examples show that default political forces can be upended. But let’s be clear on how those forces work. Yes, Republicans have a midterm turnout advantage. It’s just tempered by the penalty voters tend to levy against the party holding the White House.