House Democrats are set to begin writing significant swaths of their $3.5 trillion tax-and-spending plan on Thursday, even as new political fissures among their ranks threaten to complicate its path to passage.

The days ahead are likely to be grueling for the party’s lawmakers, who are about to embark on the tough task of translating President Biden’s broader economic agenda into law. To start, Democrats intend to focus their efforts on proposals to expand Medicare benefits, authorize new family and medical leave programs and make child care and community college more affordable.

But Democrats are starting their legislative slog at a precarious time politically, as the party’s liberal and moderate factions increasingly snipe at each other over the price tag and policy scope of their still-forming bill.

On one side are lawmakers including Sen. Bernie Sanders (I-Vt.), who initially hoped to spend even more than $3.5 trillion over 10 years — and maintains a strong preference for aggressive policy changes, particularly in areas like health care. Opposite are centrists such as Sen. Joe Manchin III (D-W.Va.), who has signaled he may support a tax-and-spending package only if it is closer to half the size of what Democrats have proposed.

Democrats are moving forward on their fall legislative agenda, with some in the party pushing for climate crisis measures as part of two key bills. (Blair Guild/The Washington Post)

The tensions spilled into public view Wednesday, less than a week after Manchin called on Congress to hit “pause” on the efforts. Senate Majority Leader Charles E. Schumer (D-N.Y.) rejected the idea in a news conference, reiterating the need for lawmakers to “all come together to get something big done.”

The early disputes threaten to complicate Democrats’ attempts to reach consensus quickly. With other potential political crises brewing — including a battle over government funding and Washington’s ability to pay its debts — the ticking clock only adds to the pressure facing the party’s leaders in September.

“It is no great secret that you’ve got 200-plus members of the House and there are disagreements there. We have 50 members in the Senate, there are disagreements there. What we are trying to do is unprecedented probably in the last 50 or 60 years,” Sanders said. “This is tough stuff.”

For Democrats, the process over the next few days to resolve their policy differences is critical. The party possesses only a slight advantage in both the House and Senate, so it cannot afford defections — even as it prepares to rely on a legislative maneuver known as reconciliation to sidestep a Republican filibuster.

But the potential for internal bickering remains high given the vast scope of the agenda that Democrats seek to adopt. That includes an expansion of Medicare benefits to cover dental, vision and hearing. House Democrats have proposed delaying the dental coverage until 2028, sparking criticism from Sanders, who said that would be too slow.

Along with changes to Medicare, Democrats also seek in reconciliation to overhaul the immigration system to help millions obtain lawful permanent resident status, including those who came to the United States as children and are often referred to as “dreamers.” And lawmakers are pursuing a major expansion of federal safety-net programs, hoping to help millions of Americans afford school, take sick and parental leave and enroll their kids in child care services. Democrats intend to pay for these and other programs through tax hikes targeting corporations and wealthy Americans.

For months, party lawmakers relied largely on substantive outlines as they made grand proclamations about the economic benefits their plans would provide. But they must now turn their early thinking into a detailed bill, a process beginning Thursday that has exposed fresh fault lines.

The primary battle concerns the package’s price tag, as Manchin and other moderates raise fears that as much as $3.5 trillion in spending could worsen the deficit and contribute to inflation. Democrats say they intend to cover the costs of their measure in full, but Manchin has maintained privately that the number still may be too high.

Instead, the West Virginia Democrat in recent days has told peers he might support only as much as $1.5 trillion package, according to a person familiar with the matter who spoke on the condition of anonymity to describe his thinking. Axios first reported on the senator’s views.

Slashing Democrats’ spending — perhaps by as much as half — would force party lawmakers to scale back some of their policy aspirations. The issue has been top of mind for lawmakers including Rep. Robert C. “Bobby” Scott (D-Va.), leader of the House Education and Labor Committee, one of the panels set to convene Thursday to begin its work on reconciliation. Entering the session, Scott’s panel has put forward a roughly $450 billion plan meant to improve the country’s child care programs, along with policies that would grant millions of Americans access to paid leave and improve aging schools.

“In this package, you are limited by the top line,” he said in a recent interview.

Privately, some Democrats have acknowledged this week they may have no choice but to whittle down their spending as they try to assuage moderates’ concerns. But aides to liberal-leaning lawmakers, who spoke on the condition of anonymity to describe their bosses’ thinking, stressed that a package as low as $1.5 trillion is essentially a nonstarter — a pledge that dooms any bill at that size.

Appearing at a news conference Thursday, House Speaker Nancy Pelosi (D-Calif.) appeared to acknowledge both of her party’s factions. “I don’t know what the final number will be,” she said. “But we have to talk about what does that take? Where would you cut child care, family medical leave paid for, universal pre-K, home health care?”

Similar intraparty squabbles have plagued the tax-focused House Ways and Means Committee, which is also set to begin its work to craft critical portions of the reconciliation bill on Thursday. Within its mandate is Medicare, setting up a fierce debate among Democrats who want to ensure that dental coverage and other benefits are delivered swiftly.

A proposal from Rep. Richard E. Neal (D-Mass.) provides vision and hearing benefits in 2022 and 2023, respectively. But it phases in the dental benefits starting in 2028, troubling Sanders and other Democrats who could attempt to tweak the proposal in the days to come.

“During the past year, 75 percent of those relying on Medicare who needed a hearing aid did not have one; 70 percent who had trouble eating because of their teeth did not go to the dentist; and 43 percent who had trouble seeing did not have an eye exam,” Rep. Lloyd Doggett (D-Tex.), who oversees the health subcommittee, said in a statement.

He added that “cost is the big concern” delaying the dental benefits — not just for the panel but for seniors as well because some Medicare beneficiaries under the proposal stand to see significant bills anyway.

Otherwise, the panel has not endeavored to lower the eligibility age for Medicare, a costly idea some Democrats have pursued. It also has yet to reveal its fuller plans for extending a series of health-care subsidies favored by Pelosi and her allies — though a Democratic aide, speaking on the condition of anonymity to describe the panel’s plans, said Wednesday that lawmakers will address the issue in the coming days. Neal’s panel also soon faces the thorniest fight of all — how to pay for as much as $3.5 trillion in new spending.

With no shortage of disputes to resolve, Democrats are running up against a fast-ticking clock in September, a month full of fiscal and economic challenges and key deadlines. Lawmakers must pass a bill before the end of the month to fund government operations to prevent a shutdown, and Democrats have said they hope to authorize additional spending in response to two recent, deadly hurricanes.

Adding to the urgency, Treasury Secretary Janet Yellen on Wednesday informed Congress that it had less time than it anticipated to address the debt limit before the U.S. government runs out of money and special measures to pay its bills. The new deadline is probably in October, she said, imploring lawmakers not to wait “until the last minute” to address the risk.

“A delay that calls into question the federal government’s ability to meet all its obligations would likely cause irreparable damage to the U.S. economy and global financial markets,” Yellen wrote in her letter to Congress.

Democrats have sought the help of Republicans to raise or suspend the debt ceiling, citing the fact that they did so to prevent a crisis under President Donald Trump. But GOP lawmakers have refused to supply the necessary votes under Biden, as they bristle over Democrats’ plans to seek as much as $3.5 trillion in new spending and tax increases they oppose.

The GOP’s resistance drew fresh criticism from Democratic leaders Wednesday, who recalled that the last stalemate over the debt limit nearly brought the country to default, rattling the global economy. At a news conference, Schumer called the Republicans’ approach a “despicable act,” adding that Democrats are exploring a “number of different ways” to address the problem.

Marianna Sotomayor contributed to this report.