WASHINGTON — A quarter-century ago, a Democratic president celebrated “the end of welfare as we know it,” challenging the poor to exercise “independence” and espousing balanced budgets and smaller government.

The Democratic Party capped a march in the opposite direction this week.

Its first major legislative act under President Biden was a deficit-financed, $1.9 trillion “American Rescue Plan” filled with programs as broad as expanded aid to nearly every family with children and as targeted as payments to Black farmers. While providing an array of benefits to the middle class, it is also a poverty-fighting initiative of potentially historic proportions, delivering more immediate cash assistance to families at the bottom of the income scale than any federal legislation since at least the New Deal.

Behind that shift is a realignment of economic, political and social forces, some decades in the making and others accelerated by the pandemic, that enabled a rapid advance in progressive priorities.

Rising inequality and stagnant incomes over much of the past two decades left a growing share of Americans — of all races, in conservative states and liberal ones, in inner cities and small towns — concerned about making ends meet. New research documented the long-term damage from child poverty.

An energized progressive vanguard pulled the Democrats leftward, not least Mr. Biden, who had campaigned as a moderating force.

Concerns about deficit spending receded under Mr. Biden’s Republican predecessor, President Donald J. Trump, while populist strains in both parties led lawmakers to pay more attention to the frustrations of people struggling to get by — a development intensified by a pandemic recession that overwhelmingly hurt low-income workers and spared higher earners.

A summer of protests against racial injustice, and a coalition led by Black voters that lifted Mr. Biden to the White House and helped give Democrats control of the Senate, put economic equity at the forefront of the new administration’s agenda.

Whether the new law is a one-off culmination of those forces, or a down payment on even more ambitious efforts to address the nation’s challenges of poverty and opportunity, will be a defining battle for Democrats in the Biden era.

In addition to trying to make permanent some of the temporary provisions in the package, Democrats hope to spend trillions of dollars to upgrade infrastructure, reduce the emissions that drive climate change, reduce the cost of college and child care, expand health coverage and guarantee paid leave and higher wages for workers.

The new Democratic stance is “a long cry from the days of ‘big government is over,’” said Margaret Weir, a political scientist at Brown University.

In the eyes of its backers, the law is not just one of the most far-reaching packages of economic and social policy in a generation. It is also, they say, the beginning of an opportunity for Democrats to unite a new majority in a deeply polarized country, built around a renewed belief in government.

“Next to civil rights, voting rights and open housing in the ’60s, and maybe next to the Affordable Care Act — maybe — this is the biggest thing Congress has done since the New Deal,” said Senator Sherrod Brown, Democrat of Ohio and a longtime champion of the antipoverty efforts included in Mr. Biden’s plan.

“People more and more realize that government can be on their side,” he said, “and now it is.”

Conservatives are hardly giving up the battle over what some call a giant welfare expansion. Democrats face high hurdles to any further ambitious legislation, starting with the Senate filibuster, which requires most legislation to get 60 votes, and the precarious nature of the party’s Senate majority. Moderate Democrats are already resisting further growth of the budget deficit.

But emboldened by the crisis, many Democrats see a new opportunity to use government to address big problems.

In addition to the new legislation being broadly popular with voters, an intensified focus on worker struggles on both the left and the right, including Republicans’ increasing efforts to define themselves as a party of the working class, has scrambled the politics of economic policy across the ideological spectrum.

Mr. Biden ran as a centrist in a Democratic Party where many activists had embraced progressive candidates like Senators Bernie Sanders and Elizabeth Warren. But he will spend the coming weeks traveling the country to promote policies like his expansion of the child tax credit, a one-year, $100-billion benefit that most Democrats hope to turn into what was once a distant progressive dream: guaranteed income for families with children.

Republicans have struggled to attack the full range of policies contained in Mr. Biden’s rescue plan, especially those like direct payments of up to $1,400 per person and expanded health care subsidies that benefit many of their constituents. Party leaders are trying to change the subject to issues like immigration.

A Republican National Committee news release this week denounced the rescue plan’s expansion of the national debt, its funding for liberal states and cities like San Francisco and $1.7 billion in aid to Amtrak, but made no mention of the expanded child tax credit that will provide most families with monthly payments of up to $300 per child.

Some prominent conservatives have welcomed the antipoverty provisions, applauding them as pro-family even though they violate core tenets of the Republican Party’s decades-long position that government aid is a disincentive to work.

Many Republicans from conservative-leaning states have turned increased attention to growing social problems in their own backyards, in the middle of an opioid crisis and economic stagnation that has left rural Americans with higher poverty rates than urban Americans, particularly for children.

An emerging strain of conservatism, often supported by a new generation of economic thinkers, has embraced expanded spending for families with children, to help lower-income workers and, in some cases, to encourage families to have more children. The conservative radio host Hugh Hewitt celebrated the expanded child credit in a series of Twitter posts on Friday, urging parents to use the proceeds to send their children to parochial school, and said he would work to make them permanent.

Still, the law could provoke a Tea Party-style backlash of the sort generated by the Obama administration’s efforts to jolt the economy back to health in 2009.

“They snuck it through and voters don’t know what they’re doing,” said Robert Rector of the conservative Heritage Foundation, an influential adviser to Capitol Hill Republicans.

“The battle has yet to be joined,” said Mickey Kaus, a journalist whose criticisms of unconditional cash benefits to the poor helped shape the welfare overhaul under President Bill Clinton.

Democrats say Mr. Biden has laid the groundwork for a durable victory by creating programs that help not just the very poor, but also lower- and middle-class workers.

The package is projected to deliver thousands of dollars in benefits to families of all races, potentially neutralizing a long history of white voters souring on spending they perceive to be targeted to racial minorities.

The rescue plan, which Mr. Biden signed into law on Thursday, features other temporary measures meant to help Americans with no or little income. They include extended and expanded unemployment benefits, increased tax breaks for child care costs and an enlarged earned-income tax credit.

Mr. Biden’s antipoverty efforts, which researchers say will lift 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.

By last summer, it became clear the pandemic’s toll was falling most heavily on disadvantaged workers, especially Black and Latino people, and Mr. Trump, who earlier had run up the deficit with a big tax cut, had joined both parties in Congress in adding trillions of dollars in federal debt to send out economic relief.

Racial protests over the summer further increased the pressure for government help. “Just as the civil rights movement pushed Johnson, this movement is pushing Biden,” said Sidney M. Milkis, a political scientist at the University of Virginia who studies the relationship between presidents and grass-roots movements.

While the expanded child tax credit would reach 93 percent of children, it would have its greatest effects on people of color. Analysts at Columbia University estimated the child benefit would 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.”