The "enthusiasm gap" could turn a Democratic wave into a tsunami

The high-turnout wins in Pennsylvania and Alabama ought to reassure Democrats — and worry Republicans — because there had previously seemed to be a pattern in which Democratic results were most impressive in low-turnout special elections. For instance, Democrat Archie Parnell surprisingly came within 3 percentage points of defeating Republican Ralph Norman in the special election in South Carolina’s 5th Congressional District, which represented a 16-point blue swing relative to the district’s partisan lean.2 But that result came on minuscule turnout — only about 88,000 votes, or 29 percent of the district’s 2016 presidential turnout. By contrast, Democrat Jon Ossoff’s underwhelming performance in the Georgia 6 runoff that same evening — which represented only a 6-point Democratic swing from the district’s partisan baseline — had come on exceptionally high turnout, equalling 79 percent of the district’s presidential vote.

After nine federal special elections that pitted the two parties against one another, however — including both the open primary3 and the runoff in Georgia 6 — there’s no overall pattern between turnout levels and how Democrats perform…