Last month, Milo Yiannopoulos, the site’s tech editor, was banned from Twitter after inspiring a sustained online harassment campaign against the “Saturday Night Live” actor Leslie Jones. Reports surfaced this week about domestic violence charges filed against Mr. Bannon stemming from a divorce in 1996.

Supporters say it is the site’s willingness to embrace viewpoints considered far outside the bounds of respectable political discourse that is the very source of its success — in the same way that Mr. Trump’s more extreme proposals, like banning Muslims from entering the country, galvanized the Republican primary electorate.

And like Mr. Trump, Breitbart excels on social media. Last month, it ranked as the 11th most popular site on Facebook, according to statistics from the social analytics firm NewsWhip. A year ago, its Facebook page had fewer than a million followers; now, it has more than 2.3 million.

The Breitbart home page drew 18 million visitors last month, roughly the same as Politico, according to data from comScore. It beat conservative competitors like The Daily Caller, though mainstream sites like CNN draw tens of millions more visitors each month.

Breitbart, which is privately owned, has declined to release revenue figures. The site appears to be backed by Mr. Breitbart’s estate; its chief executive, Larry Solov; and the family of Robert Mercer, a wealthy conservative donor and Trump supporter, according to corporate documents and two people briefed on the company’s finances.

For Mr. Marlow, the editor in chief, the site’s success comes from attracting an underserved segment of conservatives — opponents of immigration and free trade who did not see their views reflected in other outlets.