Let's get one thing out of the way—we can't possibly know with any certainty what's going to happen in 2018. The elections are just too far away, too much can change, and right now we're operating with a very small sample of information.
With that said, three partisan special elections have taken place since the January 20 inauguration of President Donald Trump. These races are as hyperlocal as you can get, and they have not drawn much attention or voter interest. But these obscure elections have followed an interesting pattern with respect to Trump.
*Virginia holds legislative elections in odd years, so these numbers are for elections in 2011, 2013, and 2015.
†The regularly scheduled election in Minnesota House District 32B was canceled in 2016 and rescheduled for the special election in February 2017.
Each of the districts shifted dramatically toward Democrats when you compare the results of the 2016 presidential election in the district to its special election results this year. Iowa's House District 89 went from 52–41 Clinton to 72–27 for the Democratic State House candidate. Minnesota's House District 32B went from 60–31 Trump to a narrow 53–47 Republican win. And Virginia's House District 71 went from 85–10 Clinton to 90–0 for the Democratic House of Delegates candidate, although it should probably be ignored because Republicans did not contest the special election. (Thanks to Daily Kos Elections for doing the invaluable work of calculating presidential results by legislative district.)
This could be evidence for the "Trump effect"—Trump's stormy tenure and record unpopularity already poisoning Republican electoral prospects as voters react to what they're seeing in the White House. Clearly, many Trump voters in these districts either didn't show up or changed parties in these special elections. However, there is an alternative explanation.
Trump's uniqueness as a Republican candidate—alienating educated whites and minorities but winning over culturally conservative Democrats—meant that the 2016 map was a departure from the previous several elections, especially in the Midwest. The Iowa and Minnesota districts that held special elections this year are two prime examples of areas that gravitated strongly to Trump. And indeed, both districts were a lot redder in 2016 than they were in 2012, when Obama won Iowa House District 89 63% to 36% and Romney won Minnesota HD-32B by just 55% to 43%.
The 2017 special election results were a lot closer to these 2012 presidential results than the 2016 ones—and even closer to the 2012 State House results (67–32 Democratic in Iowa, 51–49 Republican in Minnesota). So an equally valid hypothesis for the meaning of 2017's results is this: maybe Trump was just a one-time deal. Maybe these districts are simply reverting to their old, usual partisanship.
We can't know for sure yet which hypothesis is correct. So far, we have only run our "experiment" (i.e., special elections) to test our hypotheses in two Trumpward-moving districts, no Clintonward-moving ones (e.g., a wealthy suburban district or a minority-heavy one). Therefore, both hypotheses would predict a lurch (or return) leftward for these districts relative to 2016 presidential results. Indeed, that was the result we saw. However, we will have to wait for a special election in a Clintonward-moving district before we can differentiate between the two possibilities. Georgia's Sixth Congressional District is an excellent example, moving from 61–37 Romney to 48–47 Trump. The special general election for this seat, vacated by new Health and Human Services Secretary Tom Price, is in June.
Of course, it is also possible—even probable—that both hypotheses are partly true. A move from 52–41 Clinton to 72–27 for the Democratic State House candidate in Iowa is probably not entirely reversion to the mean when Obama won the district "just" 63–36 and Democrats captured the seat 67–32 in 2012. But rather than assuming—or fervently hoping—that these election results represent a huge anti-Trump wave building, it's good to remember that there are other possible explanations as well.