WASHINGTON — In March 2010, with Tea Party activists protesting loudly in the hallways of Capitol Hill and the political wind in their faces, 34 House Democrats — including Representative Stephen F. Lynch of Massachusetts — broke with their president to vote against passage of the Affordable Care Act.

It was not enough to kill the bill, but more than enough to register deep concerns about its reach in American society — and its potential impact on the midterm elections.

On Friday, Mr. Lynch and every other Democrat but one cast votes for about $2 trillion in spending on social welfare and climate change programs that arguably go much farther than the health law — farther, in fact, than any government intervention in half a century. And the concerns that peeled so many Democrats away from the health measure more than a decade ago were hardly in evidence — at least on their side of the aisle.

“I’ve served a couple of times in the minority, a couple of times in the majority,” said Mr. Lynch, the last remaining House Democrat who voted against Barack Obama’s signature domestic achievement. “I’ve got a better sense of time and how these moments are rare when you can seize on something and make real change.”

The political and economic shifts in the United States in the decade between the first vote and the second may explain how a party still divided could unite around legislation of such sweep. The worst public health crisis in a century laid bare economic stagnation in the middle class and the soaring wealth of the super rich. The recovery from the coronavirus crisis is still held back by child care costs and poor educational access that have kept parents home instead of working.

Searing heat waves, record wildfires and waves of battering hurricanes have underlined the reality of climate change. And the killings of Black men and women, captured on video and spread instantly around the world, raised awareness of racial injustice and inequality just as a new generation of progressives was rising up in the Democratic Party.

“I’ve always said courage comes out of crisis,” said Representative Pramila Jayapal, Democrat of Washington and the leader of the nearly 100-member Congressional Progressive Caucus. “You just couldn’t ignore this stuff anymore. It was intolerable to watch.”

Those forces appeared to unite Democrats as they made their way to the Capitol to vote. There were no shouts from angry opponents, like the ones that greeted Democrats in 2010 as they prepared to approve the Affordable Care Act. Their main barrier to passage was an eight-hour speech by the House Republican leader, Representative Kevin McCarthy of California. Beyond the chamber, the only activists in evidence were a small clutch of supporters singing old union songs and someone dressed like the Build Back Better Act, modeled after the bill in the old “Schoolhouse Rock” video.

The social safety net and climate bill still must navigate a tortuous road through the evenly divided Senate, where a single defection would bring it down. If it is able to clear that chamber, it most likely will have to go back to the House for a final vote, devoid of some of the items that drew Democrats to support it on Friday. But if it is enacted, it will touch virtually every American life, from birth to death, akin to Franklin D. Roosevelt’s New Deal or Lyndon B. Johnson’s Great Society.

Its generous subsidies for child care and universal prekindergarten are designed to lift the struggling working class into a less precarious economic position. Ample housing support, more higher education aid and worker training programs in the bill would reach well into the middle class. Home and community health care, a new hearing benefit for Medicare and price controls for prescription drugs would ease the lives of older Americans.

All of that could reasonably be described as big-government excess, and Republicans have made that case repeatedly for months. The scope and cost of the bill, if anything, is understated by its roughly $2 trillion price tag because Democrats kept the cost down by phasing in some measures and arbitrarily ending others well before the expiration of the bill’s 10-year window, some after a single year. In all, according to the Committee for a Responsible Budget, if the entire Build Back Better Act were made permanent, the 10-year cost would be $4.9 trillion, which would exceed the inflation-adjusted, four-year cost of World War II, as Mr. McCarthy charged repeatedly over eight hours.

Yet a combined assault by business organizations like the U.S. Chamber of Commerce and the Pharmaceutical Research and Manufacturers of America, their political operatives at groups like American Action Network, and the intense focus of the Republican Party against what they call “socialist” early education, crippling tax increases and dangerous limitations on the drug companies was not enough to peel away even four House Democrats, the bare minimum that would have been needed to defeat the legislation.

Competitive races next year will test whether voters embrace Republicans’ grim view of the package or Democrats’ belief that they are delivering vital programs that will be broadly appealing. Representative Tim Ryan, a Democrat from a working-class region of Ohio who is running for an open Senate seat, said Republicans have dug themselves a political hole by railing against the bill.

“They’re getting themselves in a position where they’re putting themselves against universal preschool; they’re putting themselves against controlling child-care costs for people; putting themselves on the other side of a tax cut for families,” Mr. Ryan said. “You know, people have been screaming ‘socialism’ for a long time, and Roosevelt got elected four times.”

The social welfare and climate bill is not yet law, but to have gotten it even this far was a prodigious feat, and one that probably happened only because of the societal and economic changes that preceded it, Democrats from across the spectrum of their party say.

“Look at the millennials in my district, who can’t afford a house, who can’t afford to pay back their student loans, who can’t even dream about living the lives their parents lived,” Representative John B. Larson of Connecticut, a veteran Democrat, said. “You can’t tell people they are doing better than they are.”

And as society has changed, so has the Democratic Party. Representative Jamaal Bowman’s New York district stretches from the wealthy suburb of Scarsdale to the tougher streets of Mount Vernon, and for 32 years, was represented by a white establishment Democrat, Eliot L. Engel.

Mr. Bowman, a progressive Black man who defeated Mr. Engel in a Democratic primary last year, said his constituents acknowledge wealth inequality and income stagnation in their midst — in part, because he won’t let them deny it.

“If they tried to look the other way, they really can’t,” Mr. Bowman said. “And they know that whether it’s through government or private industry, or both, we have to do more to close the inequality gap.”

Republicans are still betting that voters will fixate on the spectacle of a Democratic president struggling for months to unite his divided majority around his agenda. They argue that the chaotic and deadly withdrawal from Afghanistan, rising inflation and unsteady economic growth have only added to a sense that the country is adrift, and say they are not about to allow Democrats to use the social safety net bill to distract from those concerns.

“We have a supply chain crisis that continues to rage on, exacerbating the skyrocketing prices and scarcity of goods our citizens are experiencing,” the House Republican leader, Mr. McCarthy, said Thursday. “And ask yourself this: What has this chamber done, especially this week, to alleviate any of these pressures on our fellow citizens, the people we were sent here to represent? The answer is absolutely nothing.”

Passage of the bill, Republicans say, will only stoke inflation, force the Federal Reserve to raise interest rates, and create “stagflation,” the combination of stagnant economic growth and rising prices that doomed Jimmy Carter’s presidency.

Democrats contend that their policies will offer real relief. Economic trends indicate a need for targeted assistance to the working and middle class, and it would be paid for largely by the richest Americans and largest corporations.

Median household income grew by 41 percent from 1970 to 2000, to $70,800, at an annual average rate of 1.2 percent, according to the nonpartisan Pew Research Center. From 2000 to 2018, that annual average slowed to 0.3 percent. During that time, the 745 billionaires in the United States added $2.1 trillion to their combined net worth over during the pandemic, which now totals $5 trillion, the liberal Institute for Policy Studies recently found. And new revelations have detailed how prominent billionaires like Jeff Bezos, Elon Musk and Michael Bloomberg have paid little — and sometimes nothing — in federal income taxes.

In 2009 and 2010, the barrage of legislation that flew through Congress with huge Democratic majorities helped give rise to the Tea Party movement and a backlash against an activist government. A large-scale stimulus bill, a “cash for clunkers” measure to help people swap their old cars for new, firm new regulations to combat the Wall Street excesses that led to the Great Recession, and near-universal health care may each have had support on their own, but collectively, they appear to have overwhelmed the electorate’s tolerance for action.

This time around, Democrats have passed a $1.9 trillion pandemic aid package, a $1 trillion infrastructure bill, and are driving forward with the most far-reaching social policy since Johnson’s War on Poverty — and hoping for different political results.

“The plans we’re working to advance today are policies that are hugely popular to people of all stripes,” said Representative Matt Cartwright, a Democrat whose Pennsylvania district voted for President Donald J. Trump’s re-election by nearly 10 percentage points. “Creating bricks-and-mortar jobs for people who work with their hands, cutting taxes for the middle class, lowering the cost of prescriptions — these are things that are going to help so many people in my district.”

In 2010, a moderate Democrat was socially liberal on issues like abortion and immigration but fiscally conservative on government spending, and many of them voted against the Affordable Care Act. Today, a new brand of moderates have emerged who are fiscally populist but more circumspect on progressive social causes, like immigration, defunding police forces and pressing responses to racial injustice. They voted for the bill on Friday.

Republicans warn that Democrats will pay a steep political price for doing so. They point to the accelerating pace of retirement announcements, the latest coming on Thursday from Representative G.K. Butterfield, a senior Black Democrat from North Carolina who would face a difficult re-election if a Republican-gerrymandered district map for 2022 passes court muster.

“Who can blame these Democrats for retiring?” Mr. McCarthy asked, almost taunting them before the vote. “They see the writing on the wall, and they know this reconciliation bill will be the end of their Democrat majority, and for many, their congressional careers.”

(Mr. Butterfield, who noted he was 74 years old, said Thursday night that he appreciated Mr. McCarthy’s sense of humor.)

Mr. Cartwright, who in 2012 unseated a fellow Democrat in Northeastern Pennsylvania because he had voted against the health law, said he did not fear the Republican attacks to come.

“Republicans have opposed things that help ordinary working people for generation after generation after generation,” he said. “They spoke of downfall of civilization when Roosevelt came up with Social Security.”