The reasons for the decline appear to be both political and logistical, with lower voter enthusiasm and newly enacted impediments to voting at play. In North Carolina, where a federal appeals court accused Republicans of an “almost surgical” assault on black turnout and Republican-run election boards curtailed early-voting sites, black turnout is down 16 percent. White turnout, however, is up 15 percent. Democrats are planning an aggressive final push, including a visit by President Obama to the state on Wednesday.

But in Florida, which extended early voting after long lines left some voters waiting for hours in 2012, African-Americans’ share of the electorate that has gone to the polls in person so far has decreased, to 15 percent today from 25 percent four years ago.

The problems for Democrats do not end there. In Ohio, which also cut back its early voting, voter participation in the heavily Democratic areas near Cleveland, Columbus and Toledo has been down, though the Clinton campaign said it was encouraged by a busy day on Sunday when African-American churches led voter drives across the state.