WASHINGTON — Days before his inauguration, President-elect Biden was eying a $1.3 trillion rescue plan aimed squarely at the middle class he has always championed, but pared down to attract some Republican support.

In a private conversation, Senator Chuck Schumer, the New York Democrat who is now the majority leader, echoed others in the party and urged Mr. Biden to think bigger. True, the coronavirus pandemic had disrupted the lives of those in the middle, but it had also plunged millions of people into poverty. With Democrats in control, the new president should push for something closer to $2 trillion, Mr. Schumer told Mr. Biden.

On Friday, “Scranton Joe” Biden, whose five-decade political identity has been largely shaped by his appeal to union workers and blue-collar tradesmen like those from his Pennsylvania hometown, will sign into law a $1.9 trillion spending plan that includes the biggest antipoverty effort in a generation.

The new role as a crusader for the poor represents an evolution for Mr. Biden, who spent much of his 36 years in Congress concentrating on foreign policy, judicial fights, gun control and criminal justice issues by virtue of his committee chairmanships in the Senate. For the most part, he ceded domestic economic policy to others.

But aides say he has embraced his new role. Mr. Biden has done so in part by following progressives in his party to the left and accepting the encouragement of his inner circle to use Democratic power to make sweeping rather than incremental change. He has also been moved by the inequities in pain and suffering that the pandemic has inflicted on the poorest Americans, aides say.

“We all grow,” said Representative James E. Clyburn of South Carolina, the No. 3 House Democrat, whose endorsement in the primaries was crucial to Mr. Biden winning the presidency. “During the campaign, he recognized what was happening in this country, this pandemic. It is not like anything we have had in 100 years. If you are going to address Covid-19’s impact, you have to address the economic disparities that exist in this country.”

A vast share of the money approved by Congress will benefit the lowest-income Americans, including tax credits and direct checks, of which nearly half will be delivered to people who are unemployed, below the poverty line or barely making enough to feed and shelter their families. Billions of dollars will be used to extend benefits for the unemployed. Child tax credits will largely benefit the poorest Americans.

“Millions of people out of work through no fault of their own,” the president said moments after the relief act passed the Senate over the weekend. “I want to emphasize that: through no fault of their own. Food bank lines stretching for miles. Did any of you ever think you’d see that in America, in cities all across this country?”

The president’s closest advisers insist that the far-reaching antipoverty effort — a core tenet of the progressive wing of the Democratic Party — is less of an ideological shift from Mr. Biden’s middle-class roots than it is a response to the moment he finds himself in: presiding over a historic health crisis that has vastly increased the number of poor Americans.

They are quick to note that the president’s American Rescue Plan also directs enormous sums of money to middle-income people who have jobs but are struggling. Working families making up to $150,000 will receive direct payments, help for child care and expanded child tax credits that will bolster their annual incomes during the pandemic.

Mr. Biden is planning a public relations blitz across the country during the next several weeks to promote the benefits of the relief package and his role in pushing it through Congress. His campaign will begin on Thursday with a prime-time address from the Oval Office for the first anniversary of the Covid restrictions imposed by President Donald J. Trump.

After that, aides say Mr. Biden will travel to communities that benefit from the provisions of the new law, in part to build the case for making some of the temporary measures a permanent part of the social safety net.

Congressional Democrats are also determined to make sure the public understands what is in the new bill. In a letter sent on Tuesday to his colleagues, Mr. Schumer said that “we cannot be shy in telling the American people how this historic legislation directly helps them.”

Among the lessons Democrats say they have learned from the political backlash in 2010 to their handling of the economic crisis in 2009 is that they were not aggressive enough in selling the benefits of their stimulus package to voters a decade ago. It is not a mistake they intend to make again.

Even as Mr. Biden’s stimulus victory lap will be embraced by the left, he remains in the cautious middle so far on foreign policy, easing off on punishing the crown prince of Saudi Arabia for ordering the killing of a Washington Post journalist and imposing only modest sanctions on Russia for the poisoning and jailing of Aleksei A. Navalny, the opposition leader there.

Mr. Biden’s former Senate colleagues also acknowledge that historically he was never a driver of liberal economic policy.

Once a 29-year-old Senate candidate who pushed for civil rights and opposed the Vietnam War, Mr. Biden later drifted toward the middle, adapting to the political moment in 1996 by backing a bipartisan welfare overhaul supported by President Bill Clinton but opposed by many liberals who saw it as punitive and politically driven. Mr. Biden is now embracing a sweeping expansion of the welfare state with a price tag that is just under half of what the entire federal government spent in 2019.

“He has gotten in front of it and put his stamp on it,” said Rahm Emanuel, the former Chicago mayor and former White House chief of staff.

Tom Daschle, the former Senate Democratic leader and a longtime colleague of Mr. Biden’s, acknowledged that the president — who was the chairman of the Senate Judiciary Committee from 1987 to 1995 and the chairman of the Senate Foreign Relations Committee from 2001 to 2003 — was not a leader in those years on economic policy. But he said it was natural that Mr. Biden would aggressively tackle it now, given conditions in the country.

“Times have changed,” Mr. Daschle said, noting that “economic and racial disparities have become more acute, more understood and more important in recent years.” He pointed to the new $3,000 child tax credit, a temporary benefit included in the package, and compared its transformational potential to the Medicare program enacted under President Lyndon B. Johnson should it become permanent.

“If or when it does,” Mr. Daschle said, “Joe Biden will be seen as the L.B.J. for low-income families in dramatically improving their economic circumstances.”

During the presidential campaign, Mr. Biden spoke about “rebuilding the backbone of the nation,” a phrase that sometimes appeared to include a promise to provide significant help for people at the bottom of the economic ladder.

“Ending poverty won’t be just an aspiration, but a way to build a new economy,” he said in 2019, as he campaigned for the Democratic nomination. Once in the Oval Office, Mr. Biden hung a picture of President Franklin D. Roosevelt and invoked the Depression-era president in his private conversations with lawmakers.

The plight of the middle class has long animated Mr. Biden. He lamented their fortunes when he ran for president in 1988, during the Reagan era, and was often a lonely voice for the same constituency while serving as vice president, when he was President Barack Obama’s de facto liaison to organized labor.

To that end, Mr. Biden has also emphasized the parts of the relief package dedicated to making life easier for the working- and middle-class voters he has always courted.

“For a typical middle-class family of four — husband and wife working, making $100,000 a year total with two kids — will get $5,600, and it’ll be on the way soon,” Mr. Biden told reporters on Saturday.

But for now, his path forward is clear. Even though Mr. Biden listened politely last month when a group of Senate Republicans visited the Oval Office and pitched him on a smaller compromise deal on the relief package, he held fast to the ambitious proposal put forth by congressional Democrats. In his first major act as president, Mr. Biden leveraged the pandemic to fulfill some of the left’s longstanding goals.

Representative Pete Aguilar of California, a member of the Democratic leadership, announced at a news conference on Tuesday that the relief law “represents the boldest action taken on behalf of the American people since the Great Depression.” And Representative Hakeem Jeffries of New York, the fourth-ranking House Democrat, praised the president.

“Joe Biden has been clear that we have to go big at a moment like this,” he said.