Families are separated every day in the criminal justice and child welfare systems, often just as abruptly as at the border. Some parents struggle to support children without federal policies like paid family leave and subsidized child care that are offered in other countries.

In less visible ways, even policies and programs intended to support families can undermine them. Some families qualify for fewer federal benefits that help the working poor, like the earned-income tax credit and Medicaid, if the parents marry. Many states that prioritize health care for children offer no help to their parents, despite evidence that children have better outcomes when their parents are healthy, too.

In the child welfare system, case workers frequently neglect the role of fathers, despite mounting evidence of their importance for children’s well-being. And prison systems that hold family visitation ignore what researchers have learned about physical contact: Seeing parents through plexiglass is another form of trauma for children.

Before the 1970s, politicians seldom preached about families, according to research by the political scientists Steven Greene and Laurel Elder, who have analyzed the language used in political speeches. By 1992, conservatives were using “family values” as a motto and weapon of critique. Families became more politicized, Mr. Greene and Ms. Elder argue, as the American family itself went through major changes — with more mothers working, more single and same-sex parents, and the rise of more intensive parenting.

Over this time, the family has come to sit at the center of a core philosophical divide between the left and the right, even as both claim to care about families the most. As the left sees it, government plays an essential role protecting and supporting families, through programs like Medicaid or a higher minimum wage. To the right, it seems government too often burdens families, who need lower taxes and less regulation.