WASHINGTON — For years, they have lurked in the web’s dark corners, masking themselves with cartoon images and writing screeds about the demise of white culture under ominous pseudonyms. But on Saturday, in the wake of Donald J. Trump’s surprising election victory, hundreds of his extremist supporters converged on the capital to herald a moment of political ascendance that many had thought to be far away.
In the bowels of the Ronald Reagan Building and International Trade Center, three blocks from the White House, members of the so-called alt-right movement gathered for what they had supposed would be an autopsy to plot their grim future under a Clinton administration. Instead, they celebrated the unexpected march of their white nationalist ideas toward the mainstream, portraying Mr. Trump’s win as validation that the tide had turned in their fight to preserve white culture.
“It’s been an awakening,” Richard B. Spencer, who is credited with coining the term alt-right, said at the gathering on Saturday. “This is what a successful movement looks like.”
The movement has been critical of politicians of all stripes for promoting diversity, immigration and perceived political correctness. Its critics call it a rebranded version of the Ku Klux Klan, promoting anti-Semitism, violence and suppression of minorities.
Intellectual leaders of the movement argue that they are merely trying to realize their desire for a white “ethno-state” where they can be left alone. Mr. Trump, with his divisive language about immigrants and Muslims, has given them hope that these dreams can come true.
“I never thought we would get to this point, any point close to mainstream acceptance or political influence,” said Matt Forney, 28, of Chicago. “The culture is moving more in my direction.”
Emboldened by Mr. Trump’s takeover of the Republican Party, Mr. Forney said he expected people openly associated with the white nationalist movement to run as candidates in the 2018 midterm elections. The rise of populism and the decline of political correctness, he said, present a rare opportunity.
Robert Taylor, 29, described the conference as a “victory party.” Mr. Taylor was a committed libertarian, he said, working for Ron Paul’s presidential campaigns and even moving to New Hampshire for a project organized by the like-minded. If Hillary Clinton had won the election, he said, he would have advocated secession.
“I thought I had all the right answers and had read all the right books,” he said. “I heard about the alt-right movement, and it just lit a fire in me.”
Mr. Taylor said that with Mr. Trump, “we have breathing room; we have a little time.”
Mr. Trump has shrugged off any suggestions that he has connections to the alt-right. But his hard-line views on immigration and his “America First” foreign policy have captivated members of the movement. His appointmentas chief strategist of Stephen K. Bannon, who has called Breitbart News, the website he long ran, a platform for the alt-right, has reinforced the notion that the incoming president is on their side.
The white nationalist embrace of Mr. Trump was on display Saturday at the gathering, which was the annual conference of a group called the National Policy Institute. Guests nibbled on chicken piccata while discussing ways to reorient America’s demographics. Many of the attendees, who were mostly white men, wore red “Make America Great Again” hats. T-shirts emblazoned with Mr. Trump’s face sold quickly.
While the enthusiasm inside the conference was evident, the resistance to the alt-right remains powerful. A recent surge in hate crimes and reports of verbal and physical assaults on minorities are putting new pressure on groups that promote racism.
Many sites will not host their events, and some of their members have had their social media accounts suspended in response to vicious trolling of Jewish journalists and critics of Mr. Trump. A large group of protesters marched around the Ronald Reagan Building, which, as a federal property, could not decline to host the conference.
“These people have their right to freedom of speech, but the values they represent don’t represent America,” said Jon Pattee, 48. “I characterize them as the shirt-and-tie arm of the white supremacist-nationalist movement.”
Republicans who are more mainstream are also unlikely to accept the movement’s more provocative ideas.
“They have to grow up and start shedding some of their more controversial elements,” said Erick Erickson, a conservative blogger and commentator who has been critical of Mr. Trump. “I don’t think they will ever be accepted wholeheartedly in the Republican Party.”
Nonetheless, alt-right leaders said they planned to use their newfound influence to pressure Mr. Trump to take more “heretical” policy positions, such as a moratorium on net immigration for the next 50 years. White Europeans, Mr. Spencer said, would be given preference.
“In the long run, people like Bannon and Trump will be open to the clarity of our ideas,” said Jared Taylor, the founder of the white nationalist publication American Renaissance.
Like Mr. Trump, Mr. Spencer, who has become the face of the alt-right, derided NATO as “clumsy and ineffective.” He called for friendlier relations with the Russian president, Vladimir V. Putin, and for the deportation of undocumented immigrants, drawing chants of “build that wall.”
“I think moving forward the alt-right as an intellectual vanguard can complete Trump,” Mr. Spencer said. “We can be the ones who are out front, who are thinking about things that he hasn’t grasped yet.”
Although alt-right leaders say they want to become more politically active, it remains unclear how they will react to being more closely aligned to the establishment or what they will do if Mr. Trump starts to moderate his views. His outreach to African-Americans during the final months of the campaign angered some of his white nationalist followers, raising concerns among them that Mr. Trump might not be so different after all.
“It’s a fleeting moment of optimism,” said Al Stankard, 29, of Baltimore, who goes by the pseudonym Haarlen Venison online and was handing out his novel, “Death to the World.”
Mr. Stankard said he thought it was unlikely that Mr. Trump would be able to do things like end affirmative action, even though he believes that the president-elect sympathizes with the plight of “white racists.” He predicted that Mr. Trump might disappoint white nationalists in the same way that President Obama disappointed some of his supporters by failing to bring postracial unity to the nation.
“These are semi-delusional fantasies,” Mr. Stankard said.