WASHINGTON — President Obama is rethinking his plans to withdraw from the political arena after he leaves office next year, hinting to friends and supporters that he wants to add his voice to the shellshocked Democratic activists and elected officials who are now angrily vowing to oppose Donald J. Trump’s presidency.

White House aides say they expect the president to try to refrain from criticism during the transition because of his belief in the importance of a courteous and dignified transfer of power. But while the president holds out hope that he might influence Mr. Trump, he has made it clear that once out of office he will not remain silent if Mr. Trump goes too far in undoing his legacy.

“I’m going to be constrained in what I do with all of you until I am again a private citizen,” Mr. Obama, who will be living a few miles from the White House next year, told a meeting this past week of Organizing for Action, the group that maintains his political movement. “But that’s not so far off.”

Dozens of liberal advocacy groups, which have received a flood of donations and new members in the chaotic days since Mr. Trump’s defeat of Hillary Clinton, are gearing up for years of clashes with Mr. Trump. After eight years of advocacy on behalf of the Obama agenda, they are racing to recast themselves as bulwarks against Mr. Trump’s expected assault on an array of Democratic policies.

The mobilization against Mr. Trump began even before his victory was official. At just after midnight on Election Day, panicked immigration rights activists gathered for a conference call to strategize. A few days later, more than 80 representatives of 57 progressive groups convened in the offices of the Leadership Conference on Civil and Human Rights in Washington.

The mood was intense, angry and unforgiving, according to people who attended the confidential organizing session, which included representatives from labor, environmental groups, immigration activists, gay rights and civil rights organizations. Jim Messina, who managed Mr. Obama’s re-election campaign, offered brief remarks at the meeting, cautioning against any attempts to compromise and work with Mr. Trump.

“Push back at every level,” Mr. Messina urged. No one in the room objected, one attendee said.

Nan Aron, the president of the Alliance for Justice, started calling law firms in Washington the day after the election, beginning the process of opposition research on Mr. Trump’s likely Supreme Court nominees. Dozens of lawyers eagerly signed up for what is sure to be a heated battle over the direction of the court for a generation.

President Obama said he would be “constrained in what I do with all of you until I am again a private citizen.” Credit Stephen Crowley/The New York Times

“They want to get involved,” she said. “They are worried about the new administration.”

Neera Tanden, an adviser to Mrs. Clinton and the president of the Center for American Progress, a liberal think tank, said her organization had begun hosting daily meetings and conference calls as it plots the best way to resist Mr. Trump.

She called Mr. Trump’s first appointments extreme and said they had helped to “stiffen the spines” of Democrats. And she said the election had sparked a grass-roots awakening, with largely peaceful protests across the country.

Two planned rallies in Washington — one for immigration and civil rights on Jan. 14, and another focused on women the day after the inauguration — were devised mostly by social media campaigns. Activists in Washington expect hundreds of thousands of people to gather for the second rally.

But Ms. Tanden warned of dangers posed by the Trump administration, and warned against treating them “in normal Washington political terms.”

“I think the issue is, we do have the makings of an administration that could do more damage to democratic norms than any presidency in my lifetime,” she said.

Some Democrats say they are eager for Mrs. Clinton to re-emerge after a period of recovery interrupted so far only by a speech to the Children’s Defense Fund on Wednesday.

It is unclear when, or if, she might return to politics, though many Democrats said they would welcome it. “She’s one tough lady, and public service is in her blood,” said Representative Adam Schiff of California. “Don’t expect her to go quietly into that good night.”

But the immediate task of confronting Mr. Trump will fall to Senator Chuck Schumer of New York, the new Democratic leader.

Mr. Schumer has already alarmed some progressives in Washington with his talk of trying to cut deals with the new president on issues where their interests align. But he said on Friday that Democrats in the Senate would not hesitate to confront Mr. Trump.

“When he takes a divisive, nasty turn, when he just sides with special interests and gets co-opted by the Republican right, we will oppose him tooth and nail,” Mr. Schumer said in an interview.

That is not nearly enough for some liberal activists, who view Mr. Trump not as a traditional policy adversary to be challenged but a fundamental threat to democracy who must be confronted and destroyed.

“This is a crisis of unparalleled dimension,” said Wade Henderson, the president of the Leadership Conference on Civil and Human Rights, a coalition of liberal groups in Washington.

A disagreement over strategy threatens to sow division among Democrats, some of whom advocate finding ways to work with Mr. Trump. That approach could help senators who face re-election in 2018, especially in states where Mr. Trump trounced Mrs. Clinton.

Senator Joe Manchin III of West Virginia announced on Friday that he would support Mr. Trump’s nomination of Senator Jeff Sessions to be attorney general. Others, like Senator Richard Blumenthal of Connecticut, say Democrats should work with Mr. Trump in areas where they have similar goals (though Mr. Blumenthal also warned against ceding ground on issues like civil rights).

By contrast, Senator Elizabeth Warren of Massachusetts, whose fierce criticisms of Mr. Trump on the campaign trail often verged on the apocalyptic, rejects compromise with Mr. Trump. She says he is mounting an effort “to turn this country into something very different than it is.”

“On basic issues of humanity, we don’t give an inch,” she insisted in an interview on Friday. “Be very, very clear about what we won’t compromise on and very clear about what we’re fighting for. If we have clarity, the American people are with us.”

Senator Christopher Murphy of Connecticut reflected the uncertainty among Democrats, who are still trying to assess the implications of a Trump administration and how to respond.

“We don’t know yet if it is a threat to democratic norms,” he said, “or to policy.”

For Mr. Obama, a return to the partisan fray was never the intention. His library and foundation will serve as a platform for him to travel around the world, confront systemic issues of race relations, and push for technological change aimed at improving society.

But that vision assumed that his presidential legacy would be protected and nurtured by Mrs. Clinton in the Oval Office.

In his remarks to activists, Mr. Obama urged them to stop moping and to ratchet up their opposition to Mr. Trump by Thanksgiving. He promised to join their cause soon after, telling them: “You’re going to see me early next year, and we’re going to be in a position where we can start cooking up all kinds of great stuff to do.”

He has echoed that message in private conversations, making it clear that he may not completely pattern himself after George W. Bush, who almost never criticized his successor.

One friend of Mr. Obama’s, who requested anonymity to discuss private discussions with the president, said the election results seemed to have made him more willing to remain part of the political debate.

“Everyone he talks to walks away with this impression,” the friend said.

In an interview with The New Yorker this week, Mr. Obama said that if Mrs. Clinton had won the election, he might have just turned over the keys and walked away on Inauguration Day. With Mr. Trump’s victory, he said he felt “some responsibility to at least offer my counsel” to the Democratic Party’s political warriors he leaves behind in Washington.

Exactly how — and when — Mr. Obama would once again engage in direct and public opposition to Mr. Trump’s agenda is unclear.

“I don’t know what President Obama will do,” Ms. Tanden said. “But I know that he loves the foundations of democracy. If he thinks that’s threatened, I imagine he might speak out.”