nearly six million children out of poverty, “came to be part of the package because families that earn in the bottom third of the income distribution, or at least of the wage distribution, have been disproportionately hurt by the pandemic,” said Cecilia Rouse, the chairwoman of the White House Council of Economic Advisers.

Democrats and poverty researchers began laying the groundwork for many of those provisions years ago, amid economic changes that exposed holes in the safety net. When a 2015 book by Kathryn J. Edin and H. Luke Shaefer, “$2.00 a Day,” argued that rising numbers of families spent months with virtually no cash income, Mr. Brown arranged for all his Democratic Senate colleagues to receive a copy.

At the same time, many scholars shifted their focus from whether government benefits discouraged parents from working to whether the vagaries of a low-wage labor market left parents with adequate money to raise a child.

A growing body of academic research, which Obama administration officials began to herald shortly before leaving office, showed that a large proportion of children spent part of their childhood below the poverty line and that even short episodes of poverty left children less likely to prosper as adults. A landmark report by the National Academies of Sciences, Engineering and Medicine in 2019 found that aid programs left children better off.

“That allowed us to change the conversation,” away from the dangers of dependency “to the good these programs do,” said Hilary W. Hoynes, an economist at the University of California, Berkeley, who served on the committee that wrote the report.

cut child poverty from prepandemic levels among whites by 39 percent, Latinos by 45 percent and African-Americans by 52 percent.

“Covid exposed the fissures of systemic racism and systemic poverty that already existed,” said the Rev. William J. Barber II, who helps run the Poor People’s Campaign, an effort to get the needy more involved in electoral politics. “It forced a deeper conversation about poverty and wages in this country.”

White House officials and Democratic leaders in Congress say Mr. Biden’s rescue plan has now changed that conversation, creating momentum for permanent expansions of many of its antipoverty efforts. Multiple researchers project the bill will cut child poverty in half this year.

Democrats say they will turn that into an argument against Republicans who might oppose making the benefits permanent. “You’re voting for doubling the child poverty rate — you’re going to do that?” Mr. Brown said.

In selling the plan, Mr. Biden has blurred the lines between the poor and the middle class, treating them less as distinct groups with separate problems than as overlapping and shifting populations of people who were struggling with economic insecurity even before the pandemic. Last week, he at once talked of “millions of people out of work through no fault of their own” and cited the benefits his plan would bring to families with annual incomes of $100,000.

“This is part of why I think it is more transformational,” said Brian Deese, who heads Mr. Biden’s National Economic Council. “This is not just a targeted antipoverty program.”

In coming months, Democrats will face significant hurdles in making provisions like the child benefit permanent, including pressure from fiscal hawks to offset them by raising taxes or cutting other spending.

But the swift passage of even the temporary provisions has left many antipoverty experts delighted.

“A year ago, I would have said it was a pipe dream,” said Stacy Taylor, who tracks poverty policy for Fresh EBT by Propel, a phone application used by millions of food stamp recipients. “I can’t believe we’re going to have a guaranteed income for families with children.”

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