“It will be as close as we are ever going to have — hopefully — to a state-run media enterprise,” said Kurt Bardella, a former Breitbart spokesman who quit the site this year, saying it had turned into a de facto “super PAC” for Mr. Trump.
Breitbart has been denounced as misogynist, racist and xenophobic, and it served as a clearinghouse for attacks on Mr. Trump’s adversaries, spreading unsubstantiated rumors about Hillary Clinton’s health and undermining its own reporter, Michelle Fields, after she accused Corey Lewandowski, then Mr. Trump’s campaign manager, of assaulting her.
The site frequently boasts about knowing the pulse of its readers. News articles with evocative headlines, like “Paris Streets Turned into WARZONE by Violent Migrants,” are frequently followed by comments from readers about “the enemy within,” migrant “scum” and the “Jewish-controlled media.” Breitbart’s writers often vilify the Black Lives Matter movement, emphasizing what they say is a wave of “black-on-black crime.”
But the site’s influence on social media, where more and more Americans now consume information, has been palpable. On election night, Breitbart’s Facebook page received the fourth-highest number of user interactions on the entire platform — beating Fox News, CNN and The New York Times.
Mr. Marlow, the editor, praised Mr. Bannon on Sunday, saying, “Steve understands the voters, the American people, better than just about anyone.” But he rejected the premise that Breitbart could become an American version of Pravda.
“Our loyalty is not going to be to Donald Trump; our loyalty is to our readers and to our values,” Mr. Marlow, 30, said. “That’s regardless of what role Steve has.”
“If Trump runs his administration and honors the voters who voted him in, we’re all good,” Mr. Marlow added. “But if he is going to turn his back on those values and principles that drove his voters to the polls, we’re going to be highly critical. We’re not going to think twice about it.”
For now, Breitbart is supporting the president-elect. Its post-Election Day coverage has been, if anything, emboldened: “Meltdown Continues: Wave of Fake ‘Hate Crimes’ Sweeps Social Media,” read a headline on its home page on Sunday, attempting to cast doubt on a wave of reports of intimidation and harassment by Trump supporters. “Anti-Democracy Crybabies March by Thousands Nationwide,” read another.
The site’s expansion of political coverage comes at a time when other news outlets in Washington are concerned about staying relevant with readers — and girding for tensions with a president-elect who denounces reporters as dishonest, or worse.
A spokesman for the White House Correspondents’ Association, which coordinates press coverage of the White House, declined to comment on Mr. Bannon’s appointment.
Andrew Breitbart, the site’s founder, who died in 2012, “used to talk about the Democrat-media complex,” recalled Ben Shapiro, Breitbart’s former editor at large.
“It’s hard to think of a more Republican-media complex than Breitbart and the Trump team,” Mr. Shapiro said. “I’ll be fascinated to see if there are any points of departure, any points of criticism at all.”
Outlets like Fox News, which has a large Republican audience, insist that Breitbart is no competitor, saying that an online-only outlet with few known personalities can hardly compete with television networks that reach tens of millions of homes.
Breitbart receives far fewer unique web visitors than Fox News’s digital sites, according to statistics from comScore. Still, its Facebook audience has more than doubled in the last year, and it frequently sets the agenda for social media users with their own mass followings. The site has spotlighted nationalist views and conspiracies once relegated to the right-wing fringe.
Larry Solov, Breitbart’s chief executive, declined on Sunday to provide revenue figures for the site. Nor would he comment on whether Mr. Bannon, a former Goldman Sachs banker, retains a financial stake. (Mr. Solov is a part owner, along with Andrew Breitbart’s estate and the family of Robert Mercer, a wealthy Trump donor.)
Speaking by telephone from Hearst Castle in California, which he was visiting for a postelection vacation, Mr. Solov said that his teams had been flooded with résumés from reporters and even some aspiring journalists with no experience, “who feel motivated and energized.”
“We’ve built a community, and I really emphasize that,” Mr. Solov said. “People come to us because they feel they belong to something.”
Mr. Marlow, the editor, said the site’s international expansion was tied to upcoming elections in France and Germany. He said that Breitbart planned to support the candidacy of Marine Le Pen, the leader of France’s far-right National Front party.
“There’s an underserved readership” in Europe, Mr. Marlow said, before referring to the recent “Brexit” vote. “It’s the same readers who had been ignored in Britain and had been ignored in the United States.”
On Sunday, with Mr. Bannon elevated to one of the country’s most powerful positions, the site took on a celebratory air. Linking to a story about Mr. Bannon’s new role, the @BreitbartNews Twitter account wrote, to its more than 400,000 followers: “What a time to be alive.”