How Kamala Harris would govern

If she wins the presidency, she would take office with a relatively strong economy and the political appetite for big, expensive legislative swings severely diminished. Biden already spent trillions of dollars putting in place an agenda that encompassed many of the Democratic Party’s boldest ideas.

Now, just weeks after she rose to the top of the ticket, Harris is trying to map out an agenda that matches Biden’s ambitions, but fits into that much narrower reality—particularly if Congress is divided or fully under GOP control.

“It’s going to be very difficult for her," said Moody’s Analytics chief economist Mark Zandi. “She’s not going to have the same kind of carte blanche to spend. She can’t be as big and bold. There’s no prospect for another $2 trillion plan."

The vice president’s economic advisers are scrambling to assemble a policy agenda on the fly. Less than five weeks ago, she and her team were still expecting to play a supporting role for Biden, and they had done little to prepare for taking over the White House.

A policy-development process that typically takes presidential candidates months—or years—to get right has been truncated, with just over 70 days to go until the November election. Harris faces the challenge of charting a distinct agenda while still serving under her unpopular boss, making it difficult to disavow Biden’s policy stances.

And every new policy proposal she puts forward gives political ammunition to her opponent, former President Donald Trump. The Republican nominee has been attacking Harris for not being specific enough in her plans, though he has also avoided providing details for many of his proposals.

So far, Harris has endorsed many of Biden’s leftovers—the suite of proposals that he didn’t manage to get through Congress, from paid family leave to affordable child care. They are all ideas that Harris has long advocated for both in public and private.

She also has proposed reworked or expanded versions of other Biden policies—including a more generous child tax credit—an agenda that can be summarized as Biden 2.0-plus. Harris is planning to build on the Biden administration policies she wants to emphasize, with an economic message that seeks to better connect with voters.

“Of course, we can’t discount the fact that she’s the sitting vice president, but this is going to look different, and it is going to feel [like] a material change," Rohini Kosoglu, a Harris policy adviser, said of a potential Harris administration.

The vice president’s message on the campaign trail centers on building what she calls an “opportunity economy" in which “every American has the opportunity to own a home, start a business, and build wealth, including intergenerational wealth."

She will focus on tackling the lingering effects of the Covid-19 pandemic and inflation that are still weighing on Americans—including the high costs of groceries, housing and raising children—and that also highlight her experience and policy interests.

Housing and policies related to children have long been a focus for Harris, going back to her tenure as California’s attorney general. Harris secured a $20 billion mortgage settlement with big banks during the foreclosure crisis and established a children’s bureau within the state’s Justice Department.

Harris released a policy plan last week that set a goal of building three million new housing units in her four years in office to address the supply shortage that has contributed to high prices. She also proposed tax incentives to help first-time home buyers, including $25,000 in down-payment assistance.

White House and campaign officials reviewed polling that shows the cost of rent and high mortgage rates are a major frustration for voters, even as inflation moderates in other parts of the economy.

Harris made headlines for proposing what she called the first federal ban on price gouging for food and groceries. The plan was met with mockery from Trump and his allies, who compared it to Soviet-style price controls and dubbed her “Comrade Kamala." The proposal was also criticized by several prominent economists.

But the proposal, which Harris’s team framed as a centerpiece of her plan to lower prices, is limited in scope and likely would have little immediate effect on inflation. Harris’s team is modeling the federal ban on laws in dozens of states that ban price gouging during emergencies, such as a natural disaster, according to advisers.

Harris has been one the Biden administration’s biggest advocates of expanding the child tax credit to offer more generous payments to families with children amid the fallout from the pandemic. She is now proposing an even more sweeping version of that tax credit.

She proposed restoring the expanded child tax credit of up to $3,600 a child, which was put in place in 2021 during the pandemic and expired at the end of that year. She also threw her support behind a new further expansion of the tax credit that would provide up to $6,000 in total relief for middle- and low-income families during the first year of a child’s life.

Both tax-credit proposals would require congressional approval and could face opposition from Republicans. The issue could see movement if GOP lawmakers who have endorsed the idea reach a compromise with Democrats. Ohio Sen. JD Vance, Trump’s running mate, has called for an expansion of the tax credit.

The makeup of Congress will make or break Harris’s agenda if she wins the White House. Most nonpartisan analysts think Republicans are favored to take control of the Senate, while the House majority is up for grabs.

If the Democrats hold on to their Senate majority, they are likely to use a process called reconciliation to pass some of their top priorities. Reconciliation enables lawmakers to avoid the filibuster in the Senate and pass legislation largely along party lines, but its use is limited.

Senate Majority Leader Chuck Schumer (D., N.Y.) told reporters this week that if Harris wins, Democrats planned to focus on increasing the supply of affordable housing, cutting greenhouse-gas emissions and eliminating Trump-era tax cuts. Sen. Elizabeth Warren (D., Mass.) told delegates at the Democratic National Convention this week in Chicago that she hopes Democrats can pass legislation to expand affordable child care, address the immigration system and impose stricter gun laws.

Harris’s policy advisers are working on additional proposals that could be rolled out in coming weeks, including a plan to boost small businesses. But aides said they don’t expect her agenda to include the level of detail craved by Washington policy wonks. Voters, they say, are only interested in broad strokes.

“Regular folks don’t have time for footnotes, right?" said Chauncey McLean, president of Future Forward, a super PAC backing Harris. “They’re not looking for white papers."

Some advisers also said Harris thinks it is important to articulate a broad vision to voters and to figure out the details based on what actually can get passed in Congress. She is willing to make incremental progress if Republican lawmakers stand in the way of her most ambitious ideas.

They also noted that Biden waited until after the election—when the makeup of Congress became clear—to lay out many of the details of his Build Back Better agenda, suggesting she might do the same thing.

Harris, who is still relatively new to Washington, gets frustrated with bureaucracy and is willing at times to look for ways to bypass Congress, some aides said. As vice president, she created public-private partnerships on several issues she led, including addressing the causes of migration and to reduce maternal mortality.

A significant part of a potential Harris administration will also involve implementing laws Congress already has passed, according to advisers, including a roughly $1 trillion infrastructure package, climate change, tax and healthcare legislation and a multibillion-dollar measure to strengthen the domestic semiconductor industry.

So far, Harris and her team haven’t laid out a detailed plan for how she will pay for her proposals, which the Committee for a Responsible Federal Budget estimates could increase deficits by at least $1.7 trillion over a decade. (Trump hasn’t said how he would pay for his plans, either.)

Harris aides said she would pay for her proposals in part through tax increases on corporations and wealthy Americans that Biden proposed in his most recent budget plan. But she likely would need to identify additional revenue-raising proposals to offset the full cost of her plan.

Harris’s team said this week that she backs Biden’s proposal to increase the corporate tax rate to 28% from 21%, as well as a proposed 25% minimum tax on individuals with more than $100 million in wealth. Both proposal would face Republican opposition.

The vice president, known to be detail-oriented, deliberates over the policy specifics and often questions staff on how her proposals would affect average Americans.

One Harris adviser compared her approach to policy to that of Hillary Clinton. Both women, the adviser said, spent hours preparing for policy discussions and peppered aides with questions about how they would respond to critics who disagree with them. The adviser recalled being called out by Harris for being underprepared for a briefing.

Kosoglu, the Harris policy adviser, said at a Bloomberg event on the sidelines of the Democrats’ convention this week that the vice president compares her approach to policymaking to a friend flipping through a couple’s wedding album, looking for photos of themselves alongside the bride and groom.

“You want to know if you were in that person’s vision for what they saw that day," she said. “Do we reflect in our policy this vision of the future that others see themselves in?"

Write to Andrew Restuccia at andrew.restuccia@wsj.com and Tarini Parti at tarini.parti@wsj.com