The American political establishment reeled on Wednesday as leaders in both parties began coming to grips with four years of President Donald J. Trump in the White House, a once-unimaginable scenario that has now plunged the United States and its allies and adversaries into a period of deep uncertainty about the policies and impact of his administration.
Democrats, who will be out of power in both the White House and Congress for the first time since 2006, were particularly crestfallen that Hillary Clinton had a slender lead in the popular vote but lost in the Electoral College, a fate similar to Al Gore’s in 2000.
On campuses nationwide, students marched against Mr. Trump with signs bearing slogans like “Not my president,” and protesters in Oakland, Calif., smashed windows and set fire to garbage bins. On Wednesday night, thousands of people protested in several cities, including Chicago, Philadelphia, Seattle and New York, where demonstrators converged in Midtown Manhattan in front of Trump Tower, the home of the president-elect.
With millions of other voters euphoric at the election of a true political outsider as president, the clear divide over Mr. Trump inspired pleas of unity from his two biggest opponents, President Obama and Mrs. Clinton. At separate news conferences, they urged Americans to come together for the sake of the republic, and for the good of Mr. Trump’s presidency.
“We are all now rooting for his success,” said Mr. Obama, who planned to meet with Mr. Trump at the White House on Thursday. “The peaceful transfer of power is one of the hallmarks of our democracy. And over the next few months, we are going to show that to the world.”
Mrs. Clinton, in her first remarks to supporters after the election, said Americans owed Mr. Trump “an open mind and a chance to lead.” Choking back tears at times, she said she was “sorry that we did not win this election for the values we share and the vision we hold for our country.”
“This is painful, and it will be for a long time,” Mrs. Clinton said, standing beside her husband, former President Bill Clinton, in a tableau that underscored the end of a nearly 25-year era in which the Clintons dominated American politics.
The clash between excitement and dread was especially palpable over the likelihood that Mr. Trump, at the head of a unified Republican government, would try to reverse Obama administration policies and appoint a conservative Supreme Court justice. The House speaker, Paul D. Ryan, indicated on Wednesday that Republicans would try to repeal the Affordable Care Act, and Democrats privately began strategizing to thwart that agenda. Republicans also expanded their power in state capitals, and Democrats pledged resistance.
Foreign leaders who have had tense relations with Mr. Obama were particularly welcoming to Mr. Trump. Prime Minister Benjamin Netanyahu of Israel called Mr. Trump “a true friend” of Israel, while President Vladimir V. Putin of Russia said he hoped to have a “constructive dialogue” with him. Mr. Trump and Mr. Putin had previously exchanged warm words, to the consternation of both Democratic and Republican leaders, but Trump advisers said on Wednesday that the two leaders had not spoken by phone yet.
Chancellor Angela Merkel of Germany, whose immigration policies Mr. Trump has dismissed as “insane,” offered her cooperation but emphasized the importance of human rights, while President François Hollande of France noted that some of Mr. Trump’s views might test “the values and the interests that we share with the United States.” Mexican officials congratulated Mr. Trump but said they would not pay for his proposed border wall, as he has flatly insisted they will.
Mr. Trump’s campaign advisers said on Wednesday that he had been fielding calls from politicians like Mr. Ryan and world leaders, while also assembling a cabinet and White House team and selecting a conservative nominee for the Supreme Court vacancy. They said Mr. Trump was inclined to roll out a few cabinet nominations at a time, rather than kicking them off with one high-profile pick for a critical department like Treasury or State.
Among the candidates for cabinet secretaries and advisers are members of Mr. Trump’s inner circle, aides said, including Senator Jeff Sessions of Alabama, a crucial adviser on policy issues; Steven Mnuchin, a businessman who was Mr. Trump’s national finance chairman; Rudolph W. Giuliani, a former mayor of New York; Gov. Chris Christie of New Jersey; and Newt Gingrich, a former speaker of the House.
Advisers said Mr. Trump had also started thinking about ways to unite the country. Democratic leaders quickly embraced a policy priority that he highlighted in his victory speech: infrastructure spending.
Still, more than a third of Americans said in exit polls on Tuesday that they would be frightened of a Trump presidency. Among those who voted for Mrs. Clinton, the feeling was almost unanimous: 92 percent said Mr. Trump scared them.
Anxieties ran strong among Hispanics, African-Americans, Muslims, immigrants, women and others who had felt disparaged or demonized by Mr. Trump, who used harsh and racially charged language in ways that upended mainstream politics. The fact that Mr. Trump had been endorsed by a Ku Klux Klan newspaper, even if he rejected it, symbolized the sense of shock that he would now lead a vibrantly diverse democracy.
Alma Guel, 59, of El Paso, let out a long sigh as she started talking through the levels of emotional distress she had felt over the last 24 hours. She felt outrage, disbelief, then suspicion that the outcome was legitimate, then ashamed. She even started looking at property to buy in Mexico. Eventually, she just felt crushed.
“I was just heartbroken,” said Ms. Guel, who works in the safety department of her local electric company. “And all day today, I’ve been in a daze. I’ve never, ever been this affected by any other election.”
Many conservatives felt just as strongly — but in the opposite direction. The conservative radio host Laura Ingraham was overcome with emotion on her show Wednesday. “You’re bringing tears to my eyes,” she told the Republican strategist Ed Rollins as she beamed over the meaning of Mr. Trump’s win.
Jacob Stout, 20, who owns a small contracting business in Danville, Ky., said the result thrilled him because of the promise of bold action in Washington. “I’m excited, man. I’m not going to lie,” he said. “We’ve seen, especially the last eight years, talk but not drastic change that benefits the citizens. The idea that a citizen would be taking the presidency as opposed to a politician, I think that’s got people excited.”
His wife, Chloe Joslin, 24, was more tempered in her expectations, even though she also voted for Mr. Trump.
“Oh, my goodness, you see people who are disowning friends over who they are voting for,” said Ms. Joslin, a communications instructor. “It’s been a very heated race.”
Politicians also joined business leaders, as well as the many Americans with retirement and savings accounts, in keeping a nervous eye on the world financial markets, fearing the sort of backlash that wounded Britain after its vote in June to leave the European Union. While some business leaders worried that the nation would slide into recession, others were hopeful that Mr. Trump’s proposals of tax cuts, infrastructure spending and relaxed regulations would be welcomed by the markets, which reversed sharp declines overnight.
Political activity and reactions in both parties were in a surreal state of suspended animation as Republicans and Democrats began anticipating Mr. Trump’s moves. Mr. Ryan said at a news conference Wednesday that Mr. Trump had a “mandate” for his vision of government, but was sparing on the details of how they would work together. Mr. Ryan stopped campaigning for him last month after revelations that Mr. Trump had boasted about sexual assault.
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Mr. Ryan said that he had “spoken with Donald twice in the last 18 hours.”
“We talked about the work ahead of us, and the importance of bringing the nation together,” he said. “This needs to be a time of redemption, not a time of recrimination.”
Mr. Ryan could have been hinting at his own fate. There are more than a few restless conservatives in his conference in the House who were agitating for his ouster before the election because of his failure to fully embrace Mr. Trump. And whether that discontent will die down is far from clear.
Other Republicans who made their reservations about Mr. Trump proudly known before the election tried to be gracious, though some sounded more skeptical than optimistic.
Senator Ben Sasse of Nebraska, who became a public face of the anti-Trump faction on Capitol Hill, said he and his family had asked God to steer Mr. Trump in the right direction. “We pray that he will lead wisely and faithfully keep his oath to a Constitution of limited government,” Mr. Sasse said in a statement.
Then he promised to hold Mr. Trump to his word. “Starting today, I will do everything in my power to hold the president to his promises,” he said.
Elsewhere, the transition of power seemed to be unfolding in an orderly fashion. Word came from the Pentagon on Wednesday morning that Mr. Trump would begin receiving the same classified intelligence briefings as the president. And the defense secretary, Ashton B. Carter, said in a statement that he was committed to a smooth passing of power to the next commander in chief.
Christopher Mele contributed reporting. Giovanni Russonello contributed research.