Hillary Clinton set out 10 months ago to inspire and energize the Democratic Party, hoping to bring together the rising American electorate of black, brown, young and female voters into a durable presidential coalition. But buried beneath Mrs. Clinton’s wide-ranging and commanding victories on Tuesday night were troubling signs of a party that has not yet rallied to her call.

Democratic turnout has fallen drastically since 2008, the last time the party had a contested primary, with roughly three million fewer Democrats voting in the 15 states that have held caucuses or primaries through Tuesday, according to unofficial election results tallied through Wednesday afternoon.

It declined in almost every state, dropping by roughly 50 percent in Texas and 40 percent in Tennessee. In Arkansas, Alabama and Georgia, the number of Democrats voting decreased by between a quarter and a third.

The falloff in Democratic primary turnout — which often reveals whether a candidate is exciting voters and attracting them to the polls — reached deep into some of the core groups of voters Mrs. Clinton must not only win in November, but turn out in large numbers. It stands in sharp contrast to the flood of energized new voters showing up at the polls to vote for Donald J. Trump in the Republican contest.