Far more Americans favor Democrats over Republicans. For decades, the number of Americans identifying as Democrats or calling themselves independent but leaning Democratic has far exceeded the share of Republicans and Republican leaners. That gap has persisted, even in landslide Republican years like 1984 and 1994.

So why don’t Democrats perform better in national elections? Why have Democrats won only four of 10 presidential races since 1972?

A new report for Third Way, the moderate Democratic group, posits an answer: the ideological disconnect between liberal party activists and moderate party voters. In “Family Feud: Democratic Activists v. Democratic Voters,” Todd Eberly, a political scientist at St. Mary’s College in Maryland, examined data from the American National Election Studies and focused on the striking divide among Democrats.

In the 10 presidential elections since 1972, Democratic activists — those who attended a ...