ALLENTOWN, Pa. — When Representative Susan Wild, Democrat of Pennsylvania, accompanied Jill Biden, the first lady, to the Learning Hub, a newly established early education center whose walls were covered with vocabulary words in English and Spanish, on a recent Wednesday morning, Ms. Wild’s constituents were frank about the many unmet needs in their community.

Jessica Rodriguez-Colon, a case manager with a local youth house, described the struggles of helping families find affordable housing with rent skyrocketing. Brenda Fernandez, the founder of a nonprofit focused on supporting formerly incarcerated women and survivors of domestic violence, explained the challenges of ensuring homes were available for those who needed them.

Dr. Biden had a ready answer: “It’s a big part of the bill,” she said, turning in her seat to Ms. Wild. “Right, Susan?”

Ms. Wild quickly agreed. The sprawling $3.5 trillion social safety net and climate package that the House compiled last month would address everything raised during the discussion. It would devote more than $300 billion to low-income and affordable housing, provide two free years of community college and help set up a universal prekindergarten program that could help places like the Learning Hub, which serves about 150 children and families through Head Start, the federal program for preschoolers.

But left unmentioned was the uncertainty about whether any of that would survive and become law. A month after the House put together its bill, President Biden and Democrats in Congress have trimmed their ambitions. Facing unified Republican opposition and resistance to the cost of the measure by a handful of centrists in their party, led by Senators Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona, Democrats are now working to scale back the package to around $2 trillion to ensure its passage through a Congress where they hold the thinnest of majorities.

For Ms. Wild and other Democrats facing the toughest re-elections in politically competitive districts around the country, the ambiguity surrounding their marquee legislation makes for an unusual challenge outside of Washington: how to go about selling an agenda without knowing which components of it will survive the grueling legislative path to the president’s desk.

Polls show that individual components of the legislation — including increasing federal support of paid leave, elder care and child care to expanding public education — are popular among voters. But beyond being aware of a price tag that is already shrinking, few voters can track what is still in contention to be part of the final package, as the process is shrouded in private negotiations.

“We don’t want to be having to come back to people later and say, ‘Well, we really liked that idea, but it didn’t make it into the final bill,’ — so it’s a challenge,” Ms. Wild said. “As the bill’s size continues to come down, you may be talking about something at any given time that’s not going to make it into the final product.”

To get around Republican obstruction, Democrats are using a fast-track process known as reconciliation that shields legislation from a filibuster. That would allow it to pass the 50-50 Senate on a simple majority vote, with Vice President Kamala Harris casting a tiebreaking vote.

But it would still require the support of every Democratic senator — and nearly every one of their members in the House. Democratic leaders and White House officials have been haggling behind the scenes to nail down an agreement that could satisfy both Mr. Manchin and Ms. Sinema, who have been reluctant to publicly detail which proposals they want to see scaled back or jettisoned.

Congressional leaders aim to finish their negotiations in time to act on the reconciliation bill by the end of October, when they also hope to move forward on another of Mr. Biden’s top priorities, a $1 trillion bipartisan infrastructure bill that would be the largest investment in roads, bridges, broadband and other physical public works in more than a decade.

“As with any bill of such historic proportions, not every member will get everything he or she wants,” Senator Chuck Schumer of New York, the majority leader, wrote to Democrats in a letter ahead of the chamber’s return on Monday. “I deeply appreciate the sacrifices made by each and every one of you.”

It remains unclear which sacrifices will have to be made, with lawmakers still at odds over the best strategy for paring down the plan, let alone how to structure specific programs. The most potent plan to replace coal and gas-fired plants with wind, nuclear and solar energy, for example, is likely to be dropped because of Mr. Manchin’s opposition, but White House and congressional staff are cobbling together alternatives to cut emissions that could be added to the plan.

Liberals remain insistent that the bill — initially conceived as a cradle-to-grave social safety net overhaul on par with the Great Society of the 1960s — include as many programs as possible, while more moderate lawmakers have called for large investments in just a few key initiatives.

In the midst of the impasse, rank-and-file lawmakers have been left to return home to their constituents to try to promote a still-unfinished product that is shrouded in the mystery of private negotiations, all while explaining why a Democratic-controlled government has yet to deliver on promises they campaigned on.

“I try to make sure that people know what I stand for, what my positions are, what I want for our community,” Ms. Wild said in an interview, ticking off provisions in the bill that would lower prescription drug costs, provide child care and expand public education. “But if it’s not guaranteed, I also try to make sure people understand that, so they don’t feel like I’ve promised something that’s not going to happen.”

“That doesn’t always work,” she added. “Because you might think that something something’s in the bag, so to speak, and then all of a sudden, the rug gets pulled out from under you.”

Karen Schlegel, 71, who waited outside the center with a mix of protesters shouting obscenities and eager onlookers waiting for a glimpse of Dr. Biden, said she remained in full support of Mr. Biden’s agenda. She blamed congressional Democrats for delaying the president’s plan.

“He would be doing better if he had some support from Congress,” she said, carrying a hot pink sign professing love for both Bidens. “They better get a hustle on.”

Even Dr. Biden, as she trailed from classroom to classroom to watch the students engage in interactive color and shape lessons — and perform an enthusiastic penguin-inspired dance — avoided weighing in on the specifics of the bill.

“We already started when Joe got into office, and that’s what we’re fighting for,” Dr. Biden told the group, pointing to the $1.9 trillion pandemic relief bill that Democrats muscled through in March as evidence of the success of their agenda. “I’m not going to stop, nor is Joe, so I want you to have faith.”

For lawmakers like Ms. Wild, time is of the essence. Many Democrats are already growing wary of the prospects of beginning their re-election campaigns, before voters have felt the tangible impacts of either the infrastructure bill or the reconciliation package.

They will have to win over voters like Eric Paez, a 41-year-old events planner, who wants Democrats to deliver and has little patience for keeping track of the machinations on Capitol Hill standing in their way.

“I need to come home and not think about politicians,” Mr. Paez, said, smoking a cigarette and waving to neighbors walking their dogs in the early evening as he headed home from work near the child care center. “They should be doing what we voted them in to do.”