President-elect Joe Biden is planning to quickly sign a series of executive orders after being sworn into office on Jan. 20, immediately forecasting that the country’s politics have shifted and that his presidency will be guided by radically different priorities.

He will rejoin the Paris climate accords, according to those close to his campaign and commitments he has made in recent months, and he will reverse President Trump’s withdrawal from the World Health Organization. He will repeal the ban on almost all travel from some Muslim-majority countries, and he will reinstate the program allowing “dreamers,” who were brought to the United States illegally as children, to remain in the country, according to people familiar with his plans.

Although transitions of power can always include abrupt changes, the shift from Trump to Biden — from one president who sought to undermine established norms and institutions to another who has vowed to restore the established order — will be among the most startling in American history.

Biden’s top advisers have spent months quietly working on how best to implement his agenda, with hundreds of transition officials preparing to get to work inside various federal agencies. They have assembled a book filled with his campaign commitments to help guide their early decisions.

Biden is planning to set up a coronavirus task force on Monday, in recognition that the global pandemic will be the primary issue that he must confront. The task force, which could begin meeting within days, will be co-chaired by former surgeon general Vivek H. Murthy and David Kessler, a former Food and Drug Administration commissioner.

There has also been a recognition of those around him that he may have to lean more on executive actions than he had once hoped. He can reorient various federal agencies and regulations, and he can adopt a different posture on the world stage.

But pushing major legislation through Congress could prove to be a challenge.

Although the Democrats will hold a narrowed majority in the House, the final makeup of the Senate is not yet clear. That will be decided on Jan. 5, with two runoff elections in Georgia. Democrats would need to win both races to effectively have control of the Senate — with Vice President Kamala D. Harris serving as the tie-breaking vote — while Republicans would retain a narrow advantage by winning at least one.

“The policy team, the transition policy teams, are focusing now very much on executive power,” said a Biden ally who has been in touch with his team who, like others interviewed for this story, spoke on the condition of anonymity to discuss private conversations. “I expect that to be freely used in a Biden administration at this point, if the Senate becomes a roadblock.”

A Republican-held Senate — or even one with a narrow Democratic majority — probably will affect Biden’s Cabinet picks given the Senate’s power to confirm nominees.

One option being discussed is appointing Cabinet members in an acting capacity, a tactic that Trump also used.

“Just by virtue of the calendar and how many positions are filled, that’s always a possibility,” the person said. “Because the Senate moves so slowly now, so much more slowly than it used to.”

On Saturday afternoon, about two hours after networks called Biden the winner of the election, the president-elect had a brief call with House Speaker Nancy Pelosi (D-Calif.) and Senate Minority Leader Charles E. Schumer (D-N.Y.), who called to congratulate Biden on a “tremendous victory,” according to two Democratic officials.

Schumer called while en route to a celebration in Brooklyn, holding his flip phone out the window so that Biden could hear the cheering crowd.

If Sen. Mitch McConnell (R-Ky.) stays as majority leader, he would be trying to manage a conference torn between two factions with different interests, but neither necessarily eager to help Biden — one with senators running for reelection in swing states in 2022, and another with those seeking the national spotlight as they vie for the 2024 Republican presidential nomination.

“In the old days, the mandate meant that the other side would be more amenable, or feeling they had an impetus to work,” said Sen. Robert P. Casey Jr. (D-Pa.). “I’m not sure that applies any longer.”

It is unclear whether Biden has communicated with McConnell yet directly; aides have not commented on any conversation.

A closely divided Congress could hamper Biden’s efforts to do sweeping legislative actions on immigration changes. He has also said he would send a bill to Congress repealing liability protections for gun manufacturers, and close background-check loopholes. He has pledged to repeal the Republican-passed tax cuts from 2017, an effort that could be stymied if Republicans hold the Senate majority.

Without congressional cooperation, however, Biden has said that he plans to immediately reverse Trump’s rollback of 100 public health and environmental rules that the Obama administration had in place.

He would also institute new ethics guidelines at the White House, and he has pledged to sign an executive order the first day in office saying that no member of his administration could influence any Justice Department investigations.

Biden has long pledged to rejoin the Paris climate accords by executive order, but he has also said that he would attempt to persuade other nations to adopt higher standards in an attempt to curb the impacts of climate change.

Sen. Christopher A. Coons (D-Del.), a longtime Biden ally who holds the seat Biden had for 36 years, offered a broad overview of Biden’s initial agenda: “Get us out of this pandemic that’s been made far worse by Trump’s bungled mishandling of it, rebuild our economy in a way that’s more sustainable and more inclusive, and deal with division and inequality.”

He noted that Biden’s style will be quite different, saying that Trump and Pelosi haven’t spoken in more than a year.

Coons suggested that Biden would promptly begin reaching out to leaders in both parties.

The coronavirus response has been foremost on Biden’s mind, and it is seen inside his campaign as a chief reason for his victory. He has previously said that even before the inauguration he would reach out to Anthony S. Fauci, the country’s top infectious-disease expert, asking him for advice.

Biden also wants to quickly appoint a supply commander to oversee production and distribution of testing — and, when ready, vaccines — as well as materials such as masks and gowns.

The coronavirus — and Biden’s response to it — could also significantly impact the traditional spectacle that surrounds the transfer of power. Inaugural balls could be altered. And while Biden has previously said he wouldn’t envision wearing a mask while being sworn in, he has said they could try to limit the traditional throngs that fill the steps of the U.S. Capitol.

Much of Biden’s early agenda — including which pieces of legislation to prioritize — will be determined in the coming weeks as his transition team begins taking on a far more prominent role.

Biden’s transition effort is being overseen by Ted Kaufman, one of his closest advisers. Kaufman, who was appointed to replace Biden in the Senate when Biden became vice president in 2009, also helped co-write an update to the law governing the transition process, which was passed in 2015 and signed by President Barack Obama.

Biden’s transition team has been given government-issued computers and iPhones for conducting secure communications, and 10,000 square feet of office space in the Herbert C. Hoover Building in Washington, although most of the work is being done virtually because of the coronavirus pandemic. His advisers have been granted temporary security clearances and undergone FBI background checks to fast-track the processing of personnel who can receive briefings on intelligence.

But one important next step is for the head of the General Services Administration to rule that the election results are final, enabling Biden’s transition team to expand its work and gain access to government funds. Biden officials are prepared for legal action if that administrator — Emily W. Murphy, a Trump political appointee — delays that decision, according to officials familiar with the matter.

Trump has so far not conceded defeat, falsely claiming Saturday that he won the election.

Pamela Pennington, a GSA spokeswoman, said that Murphy would ascertain “the apparent successful candidate once a winner is clear based on the process laid out in the Constitution.” Until that decision is made, she said, the Biden transition team would continue to receive limited access to government resources.

The transition from Trump to Biden would have few historic parallels, rivaled perhaps only by 1860-1861, when southern states seceded before Abraham Lincoln took office, and 1932-1933, when Herbert Hoover sought to undermine Franklin D. Roosevelt and prevent him from implementing his New Deal policies.

The last time there was a prolonged delay in a transfer of power was in 2000, when uncertainty over the results in the contest between then-Vice President Al Gore (D) and then-Texas Gov. George W. Bush (R) stretched out until the Supreme Court ended a Florida recount that gave Bush the victory on Dec. 12.

The Bush administration’s sluggish start and lack of qualified personnel in place was cited by the 9/11 Commission Report as a critical vulnerability to U.S. national security for the attacks that occurred less than eight months after the inauguration. That prompted changes to the law and the granting of access at an earlier date following the political conventions.

“When George W. Bush left he made clear to his Cabinet that this is going to be the best transition of power that’s ever occurred. Because we weren’t treated very well when we came into power,” said Michael Leavitt, who at the time was the outgoing secretary of Health and Human Services. “Barack Obama to his credit said the same thing. There was a spirit of cooperation that went on and needs to continue. Whether it will or not I don’t know. But we’re better prepared.”

Chris Lu, the executive director of the Obama-Biden transition in 2008, said that within two hours of the election being called in 2008 he had a formal letter beginning the transition process.

“We literally at 9 a.m. the next morning walked into a transition office and had access to it,” he said. “It was the model for the smoothest transition of power.”

Making a clear break from the Trump administration's adversarial posture toward the civil service is also a top priority for the Biden transition team.

The Trump administration's suspicion of career officials and early calls for them to “get with the program” or “go” created tensions with incoming political appointees that never dissipated. Biden officials are hoping to create a positive atmosphere by avoiding some of the terminology and labels they think contributed to the mistrust.

The teams of campaign staffers and other aides that first embed themselves into government agencies after an election have historically been called “landing teams” and “beachhead teams,” summoning the memory of the storming of Normandy during World War II.

To avoid any associations with war, some Biden aides are sticking to soberingly bureaucratic terms, referring to landing teams as “ARTs” or Agency Review Teams, and beachhead team members as “temporary employees.”

So far, Trump administration officials have reviewed succession plans for department officials, planning for which civil servants would take on acting roles amid vacancies. Briefing materials are slated to be delivered over the next several days to Biden’s transition team.

Leavitt, who oversaw transition planning in 2012 for Republican nominee Mitt Romney and has worked with Kaufman to change the law governing presidential transitions, said there are a range of moves the Biden team could make even without cooperation from Trump’s campaign. Cabinet members and other top White House staff could be picked, and key priorities for the start of the administration could be lined up.

“The current moment always seems like it’s the extreme, and often they are. But we get through them. The country survives,” he said. “The internal strength of the United States allows us to get through these things.”

John Hudson contributed to this report.