RALEIGH, N.C. — The decline of marriage over the last generation has helped create an emerging voting bloc of unmarried women that is profoundly reshaping the American electorate to the advantage, recent elections suggest, of the Democratic Party. What is far from clear is whether Democrats will benefit in the midterm contests this fall.

With their Senate majority at stake in November, Democrats and allied groups are now stepping up an aggressive push to woo single women — young and old, highly educated and working class, never married, and divorced or widowed. This week they seized on the ruling by the Supreme Court’s conservative majority, five men, that family-owned corporations do not have to provide birth control in their insurance coverage, to buttress their arguments that Democrats better represent women’s interests.

But the challenge for Democrats is that many single women do not vote, especially in nonpresidential election years like this one. While voting declines across all groups in midterm contests for Congress and lower offices, the drop-off is steepest for minorities and unmarried women. The result is a turnout that is older, whiter and more conservative than in presidential years.

Half of all adult women over the age of 18 are unmarried — 56 million, up from 45 million in 2000 — and now account for one in four people of voting age. (Adult Hispanics eligible to vote, a group that gets more attention, number 25 million this year.) Single women have become Democrats’ most reliable supporters, behind African-Americans: In 2012, two-thirds of single women who voted supported President Obama. Among married women, a slim majority supported Mitt Romney.