Liberals now seem to acknowledge the downsides of the retreat from marriage. A report on strengthening families that was released in January by the liberal Center for American Progress recommended not only economic assistance but also social support, such as couples’ counseling services and visits by specially trained nurses and other professionals to the parents of young children.

The growth of legal same-sex marriage has made it possible for liberals to endorse the importance of marriage without feeling that they have abandoned their commitment to equality. Same-sex couples are seizing the opportunity to marry in large numbers: According to American Community Survey data analyzed by the demographer Gary J. Gates of the Williams Institute at the University of California, Los Angeles, School of Law, 34 percent of all same-sex couples in the Northeast in 2013 were married. Far from undermining heterosexual marriage, as its opponents warned, same-sex marriage has broadened support for marriage beyond its conservative base.

If the truce leads to legislation, tax credits might be a possibility. Conservatives such as Senators Rubio and Lee support an expanded tax credit for parents who are raising children. Representative Paul D. Ryan, Republican of Wisconsin, and others are backing an extension of the earned-income tax credit to childless adults which, they hope, will increase their incentive to work, and help them marry and start families.