WASHINGTON — Five House committees on Thursday will begin formally drafting their pieces of Democrats’ far-reaching social policy and climate change bill that would spend as much as $3.5 trillion over the next decade — and raise as much in taxes and other revenue boosters — to reweave the social safety net and move the country away from fossil fuels.

The products of the drafting sessions, which could take several arduous days, are to be folded into a final bill later this fall that could be one of the most significant measures to reach the House floor in decades.

“What I want people to know is that this bill is for you,” Speaker Nancy Pelosi, Democrat of California, said on Wednesday. “If you’re a woman with children at home and want to return to the work force; if you have people with disabilities at home and in home health care; if your children are little and you want universal pre-K; children learning, parents earning; if someone is sick in your family and you need family and medical leave, paid; the list goes on.”

Democrats plan to push through the legislation using a process known as reconciliation, which shields fiscal measures from filibusters and allows them to pass with a simple majority if they adhere to strict rules. The maneuver leaves the party little room for defections given its slim margins of control in Congress.

Republicans are unified in opposition to the emerging bill, and lobbyists for business and the affluent are also arrayed against it. They need only to peel away three or four House Democrats — or a single Senate Democrat — to bring the effort down.

“This week, as Democrats try to ram through their reckless $3.5 trillion tax-and-spend agenda, let’s not forget that American families and Main Street businesses will be left shouldering the burden of these devastating tax hikes,” said Republicans on the House Ways and Means Committee, which will begin drafting its hefty portion of the bill on Thursday, Friday and into next week.

The panel will start with the spending side this week before moving next week to the more difficult task of tax increases to pay for it. Among the items on its voluminous agenda: providing up to 12 weeks of paid family and medical leave; expanding tax credits to pay for child care and elder care; raising the wages of child care workers; requiring employers to automatically enroll employees in individual retirement accounts or 401(k) plans; and expanding Medicare coverage to include dental, vision and hearing benefits.

The Education and Labor Committee’s portion of the bill, also under consideration on Thursday, would make prekindergarten universal for 3- and 4-year-olds; fund two years of tuition-free community college and increase the value of Pell Grants; provide money to rebuild and modernize school buildings; expand job training programs; and extend child nutrition programs bolstered on an emergency basis during the pandemic.

The Committee on Natural Resources, which has partial purview over climate change programs, will try to raise the fees for fossil fuel companies that explore and drill on public lands and waters; expand leasing of offshore sites for wind energy; spend up to $3.5 billion on a new civilian and tribal climate corps; and boost funding for wildfire control, climate resilience and adaptation to a warmer planet.

Smaller pieces of the bill will be drafted by the science and small business committees.

Senate Democrats, who are expected to skip the public drafting phase, have been meeting behind closed doors to try to work out their version of the bill and bring it directly to the floor.

They plan to submit a proposal to the Senate’s top rule enforcer as early as Friday that would legalize several groups of undocumented immigrants, including those who were brought to the country without authorization when they were children. It is up to the parliamentarian to determine whether specific measures qualify under Senate rules to be included in the final bill, which is supposed to be restricted to policies that directly affect government revenues.

Emily Cochrane contributed reporting.