In what could be the most consequential stretch of his presidency, Joe Biden faces an autumn sprint to advance a once-in-a-generation expansion of the social safety net.

In the coming weeks, Biden and Democrats on Capitol Hill will attempt to steer the president’s multi-trillion dollar economic vision through Congress and into law. With a narrow window for action, they will have almost no room for error.

If they fail, the party will face voters in 2022 with little to show for two years in control of Congress.

If successful, Democrats will deliver a legacy-defining legislative victory, echoing the New Deal under Franklin Delano Roosevelt and Great Society under Lyndon Johnson.

“I believe this is a moment of potentially great change,” Biden said last week. “This is our moment to deal working people back into the economy.”

He spoke a day after House Democrats finished shaping a mammoth piece of legislation they hope can make it through Congress. Pursuing a perilous two-track approach, they are tying a $1tn bipartisan infrastructure deal to a $3.5tn party-line package that contains Democratic policy priorities.

The infrastructure bill passed the Senate last month with unusual bipartisan support. Democrats are attempting to secure their spending package using a special process, reconciliation, which will shield it from being blocked by Senate Republicans.

Accomplishing this will require the vote of every Democrat in the 50-50 Senate and nearly every Democrat in the House. That gives each member enormous leverage.

Two Democratic senators have objected to the size of the social spending plan, which would dedicate $3.5tn over a decade to expand healthcare and childcare and combat the climate crisis. In the House, an arrangement between party moderates and progressives has been likened to “mutually assured destruction”.

The fragile alliance is scheduled to face its first test on 27 September, when the House is expected to vote on the $1tn bipartisan bill to fund roads, broadband and other infrastructure projects. Progressives have vowed to derail the measure if they are not given ironclad assurances that the mammoth policy bill will also reach Biden’s desk.

The legislative battle will challenge Biden, both as the leader of a factious party and as an experienced congressional dealmaker. Last week, as Congress faced a dizzying array of fiscal and legislative deadlines to avert a government shutdown and a debt default, he signaled his readiness to wade deeper.

The president held “productive” meetings with the two senators concerned by the policy package, Joe Manchin of West Virginia and Kyrsten Sinema of Arizona. Later, he called the Senate majority leader, Chuck Schumer, and the House speaker, Nancy Pelosi.

Then, on Sunday, as if perfectly to illustrate the treacherous ground on which Biden must walk, the website Axios reported that Manchin wanted the spending bill paused until the new year.

Jonathan Alter, the author of books on FDR, Obama and most recently Jimmy Carter, said that after 36 years cutting deals in the Senate and eight leading legislative negotiations as vice-president to Barack Obama, Biden was uniquely qualified to play his new role. The difference is that it is his presidential legacy on the line.

“The only true power that an American president has, as the great political scientist Richard Neustadt put it, is the power to persuade,” Alter said. “In the next few weeks, we’re going to find out how much power he has.”

‘What it means to be a parent’

The intensity of the debate largely reflects the ambition of the spending proposal. Bernie Sanders, the Vermont senator overseeing the package as chair of the budget committee, has called it “the most consequential legislation since the 1930s and FDR and the New Deal”.

On the opposite end of the spectrum, Karl Rove, once chief strategist for George Bush, warned Republicans the legislation would create a “cradle-to-grave welfare state”.

The plan would touch nearly every aspect of life. It contains major initiatives on childcare, social welfare, education, healthcare, immigration, labor and climate change, financed in large part by tax increases on corporations and the wealthiest Americans.

Biden has argued that democracy itself is on the line. The spending and infrastructure bills, he has said, offer a generational opportunity to demonstrate that “American democracy can deliver” better than autocracies like China.

Progressives have been encouraged by Biden’s apparent rebuke to free-market and limited-government practices that have held sway since the days of Ronald Reagan.

Suzanne Khan, managing director of research and policy at the Roosevelt Institute, a thinktank that advocates for progressive economic policy, said the legislation at hand could be “truly transformative” for millions.

“Universal pre-K, the child tax credit, free community college – those are all visible policies that really change the economics of what it means to be a parent in the United States,” she said.

But some economists, nearly all Republicans and even a few Democrats are concerned about injecting trillions of dollars into the economy months after a coronavirus relief package did the same thing.

Michael Strain, an economist at a right-leaning thinktank, the American Enterprise Institute, warned that the spending package could undermine economic recovery.

“If you push the economy too hard and raise consumer prices, you’re actually doing a disservice to lower-income households by reducing the purchasing power of the income that they have,” he said. “From that perspective, I believe the reconciliation package will be counterproductive.”

Last week, Biden said job growth and other economic gains were due in large part to the federal relief bill, which was credited with reducing poverty during the worst economic crisis since the Great Depression. But Biden has argued that more is needed, to lift structural barriers that have held back many, particularly women and people of color.

In a speech last week, the treasury secretary, Janet Yellen, said affordable childcare was “as essential as infrastructure or energy”.

The first woman to lead the Treasury in its 232-year history said she was “not sure whether I would be here, in this job today, if I didn’t have an excellent babysitter 40 years ago”.

Activists are working to build support. Polling shows the spending plan is popular, including among independents and a notable number of Republicans. During August, a coalition of progressive groups set out to sell the plan. They were surprised to find little organized opposition.

“People want these reforms,” said Stephanie Cutter, an adviser to Building Back Together, an outside group promoting Biden’s agenda.

“They want jobs, they want better roads and bridges, they want safer drinking water, they want lower costs for childcare, paid leave, a middle-class tax cut. All of those things will impact – for the better – middle class families and they want these things. That’s why they’re so popular and that’s why they’re so hard to fight against.”

‘Good policy is good politics’

Nonetheless, the fight will be fierce. Deep-pocketed business groups are prepared to spend millions opposing changes to the tax code while Republicans attack Democrats’ spending plans.

“If you own a home, if you drive a car, if you breathe air – get ready for more taxes and more spending,” warned Senator Lindsey Graham, the ranking Republican on the budget committee.

Wrapping the bulk of Biden’s domestic agenda into one package is a risk. Internal disputes could stall or derail the bill.

Last week, three Democrats on the House energy and commerce committee joined Republicans in voting down a proposal, opposed by the pharmaceutical industry, to give Medicare the power to negotiate prescription drug prices.

The setback underscored the challenges ahead.

“The best shot Democrats have is to understand that good policy is good politics,” Sanders told NPR. “That when you stand up for working families, when you stand up to protect our kids and future generations from the devastation of climate, that not only is that the right thing to do – the American people will reward you.”