The “care economy” — a broad set of policies aimed at helping parents and other caregivers — was the great unfinished work of President Biden’s domestic agenda. Vice President Kamala Harris has made it a central aspect of her campaign to succeed him.

Ms. Harris, the Democratic nominee, has spoken frequently on the campaign trail about making it more affordable to raise children. She chose a running mate, Gov. Tim Walz of Minnesota, whose signature policy accomplishments include the creation of a paid family leave program.

In the first major economic speech of her campaign, she proposed restoring an expanded child tax credit and called for a new $6,000 benefit for parents of newborns. She also laid out policies that aim to reduce housing costs, such as providing up to $25,000 in down-payment assistance to first-time home buyers.

In her speech accepting the Democratic nomination on Thursday, Ms. Harris said she would not let conservatives end programs like Head Start that “provide preschool and child care for our children.”

But Ms. Harris has not yet offered specific proposals on child care, paid family leave or early childhood education. That has surprised some progressive policy experts, and brought flashbacks of the Biden administration’s inability to enact more sweeping policies.

Mr. Biden also initially made the care economy a central piece of his domestic policy agenda, putting it alongside proposed investments in roads and bridges, domestic manufacturing and green energy. His aides often argued that care was a form of infrastructure — that affordable child care, like highways, was essential to a well-functioning economy.

But while Mr. Biden succeeded in passing billions of dollars in investment in more traditional infrastructure projects, most of his care-oriented proposals fell by the wayside amid opposition from Republicans as well as members of his own party. His initial proposal for 12 weeks of guaranteed paid leave, for example, was whittled away to four weeks during negotiations before ultimately being cut from the plan altogether after pushback from Senator Joe Manchin III, then a conservative Democrat from West Virginia.

“Paid leave has just been such a hard thing to get passed at a federal level in this country,” said Maya Rossin-Slater, a Stanford University health economist who is a leading scholar of such programs. “It’s always this piece of legislation that gets compromised away.”

So far, progressive leaders have said they trust Ms. Harris to make care-related policies a priority, either during the campaign or, if she wins, as president. And they praised the economic proposals she offered last week.

“I really read this proposal as a ‘first 100 days’ agenda about bringing down the cost of living through investments in things that have been throwing families’ budgets into chaos for years,” said Suzanne Kahn, a vice president of Roosevelt Forward, a progressive advocacy organization. “I don’t think it’s the last policy proposal or the only policy proposal we’re going to see.”

Ms. Harris has fully embraced one key policy aimed at reducing costs for families with children: an expanded child tax credit. She has called for increasing the maximum credit to $3,600 per child — up from $2,000 now — and making it available to poorer families who currently do not qualify for the full benefit. That would restore the temporary expansion put in place as part of Mr. Biden’s 2021 pandemic relief package.

“We know this works and has a direct impact on so many issues, including child poverty,” Ms. Harris said last week.

Research has found that the credit’s temporary expansion helped millions of Americans pay for essential costs, such as food, rent and clothing. It also helped slash child poverty to record low levels. Overall, child poverty in 2021 was cut nearly in half from 2020 — then rose again when the credit expired in 2022.

Ms. Harris has also proposed a $6,000 tax credit that would provide relief for middle- and low-income families with children in the first year of life, when expenses are particularly high. The credit would aim to offset some of those costs and provide assistance to parents who have to forgo income as they take time off from work, according to the Harris campaign.

Megan Curran, the director of policy at the Center on Poverty and Social Policy at Columbia University, said an expanded child tax credit could be a “game changer for kids.” And while she noted that there were few details about how the $6,000 credit would work, she thought it would be beneficial for the federal government to provide more resources to families with newborns.

“We tend to leave families hanging at this most critical time,” Ms. Curran said.

Expanding the child tax credit enjoys a measure of bipartisan support. Senator JD Vance of Ohio, the Republican vice-presidential nominee, has floated the idea of a $5,000 credit, although he has provided few specific details about who would qualify. The House of Representatives voted overwhelmingly this year to approve a bipartisan tax deal that included a more modest expansion of the tax credit while also renewing several tax breaks aimed at businesses.

That effort failed in the Senate, however. And Ms. Harris’s proposals, too, will face a number of obstacles even if she is elected.

The first is cost. The Harris campaign has said her overall plan would reduce projected federal deficits, largely by raising taxes on the wealthiest Americans and large corporations. But her plan is bound to draw pushback from Republicans and even some moderate Democrats, who are concerned about federal spending.

Michael R. Strain, an economist at the American Enterprise Institute, a conservative think tank, said there were not enough policy details to know how much the credit’s expansion would cost, but he noted that the United States has been running huge deficits for several years.

“The debt is on an unsustainable path, and that’s something that I think we need to take very seriously,” Mr. Strain said.

Those concerns will only grow if Ms. Harris eventually proposes new federal subsidies for child care, paid family leave or early childhood education. The Biden administration estimated in 2021 that its versions of those programs would cost hundreds of billions of dollars over a decade.

Supporters of such programs argue that they pay long-term dividends and even lead to increased tax revenue as children grow into healthier, better-educated and ultimately more productive workers. But those benefits play out over decades, far beyond the 10-year period typically considered during budget discussions, let alone the four-year presidential campaign cycle.

Beyond cost, policies like universal preschool and subsidized child care could prove difficult to devise and carry out. Even some progressives, for example, criticized Mr. Biden’s initial child care plan as unworkable because it would have led to increased demand for care without doing enough to increase supply.

On the campaign trail, Ms. Harris has repeatedly said she would “fight for a future” with affordable child care and paid leave, but has offered few details on how she would grapple with the thorny issues involved.

So far, however, supporters of care policies seem willing to give Ms. Harris a pass on the lack of specificity. Many progressives say they are especially encouraged by Ms. Harris’s choice of Mr. Walz, given his work advancing similar policies in Minnesota.

They also say they are pleased by the way Ms. Harris and Mr. Walz have put child care alongside issues like housing and health care as part of a broader message about improving affordability for middle-class families. That framing is no accident: Voters routinely cite the cost of living as a top economic concern.

“More than anything, I think what it says to me is that she’s hearing about the breadth and depth of the challenges that families face,” said Jocelyn Frye, the president of the National Partnership for Women & Families.