“There’s never been a worse coalition for the purpose of a midterm election,” said David Wasserman of The Cook Political Report. The ability of Democrats to counter the problem will help determine how politically difficult Mr. Obama’s last two years in the White House could become.

Turnout for American presidential elections (roughly six in 10 eligible voters in recent contests) is always higher than for midterm congressional elections (fewer than half). But the rate of falloff from one to the next varies for different groups of voters.

In 2012, exit polls showed, Mr. Obama won a second term by rolling up margins of 11 percentage points among women, 24 percentage points among voters ages 18 to 24, and 87 percentage points among blacks. Over the last four midterm elections, turnout by all three groups fell more from the previous presidential race than turnout by Republican-leaning men, whites and those over 65.

Young voters have abandoned the midterm electorate at more than twice the rate of seniors. Hispanics, who favored Mr. Obama by a margin of 44 percentage points, have voted at just two-thirds the rate of whites. Unmarried women, the source of the Democratic advantage with women, vote less often than their married counterparts.