One lesson for Democrats would therefore seem to be to look at a mix of indicators for the competitiveness and partisanship of a district, rather than focusing on the 2016 presidential result alone. Trump’s popularity will be a key factor, but so could the long term partisan lean of the district and how it has voted for Congress in the past. Local issues, particularly how the new health care bill might affect the district, could also play a role.

Each of the special elections so far have also come with their quirks: For instance, Georgia 6 had a very high turnout and tens of millions of dollars invested by each party, whereas South Carolina 5 had a much lower turnout and very little investment. But for that very reason — because individual races can be determined by flukish and unpredictable circumstances — Democrats would be wise to avoid the mistake they made in 2016, when Clinton campaigned in too narrow a range of states and didn’t properly consider the uncertainty in the outcome. Well-educated Sun Belt districts such as Georgia 6 could be the Democrats’ path back to a majority. But so could places such as South Carolina 5 that had once been more Democratic, or districts in Ohio or Pennsylvania or upstate New York. It’s much too early to know.