Bannon provoked Rebekah Mercer's ire by making critical comments about Trump and his family to author Michael Wolff in a book, "Fire and Fury," published just last week.
Bannon is quoted in the book as saying that Trump's son, Donald Jr. and son-in-law, Jared Kushner, engaged in "treasonous" behavior by secretly meeting with Russian representatives during the campaign to get unflattering information about Trump's opponent, Hillary Clinton.
Trump replied to Bannon's comments with a statement savaging his former confidant. "Steve Bannon has nothing to do with me or my presidency. When he was fired, he not only lost his job, he lost his mind," the president said. He later attacked Wolff and the book in a tweet in which he referred to Bannon as "Sloppy Steve."
Mercer weighed in with a rare statement of her own on Thursday that distanced her from Bannon.
"I support President Trump and the platform upon which he was elected," she wrote. "My family and I have not communicated with Steve Bannon in many months and have provided no financial support to his political agenda, nor do we support his recent actions and statements."
Although Bannon continued to chair Breitbart's editorial meetings and host its satellite-radio program, Mercer's comments appeared to signal his end, people at the media company said. Breitbart's readers seemed to side with Trump in the spat.
In statements carried on Breitbart's website, Bannon said, "I'm proud of what the Breitbart team has accomplished in so short a period of time in building out a world-class news platform."
The site's chief executive, Larry Solov, said, "Steve is a valued part of our legacy, and we will always be grateful for his contributions, and what he has helped us to accomplish."
The Mercers were largely responsible for Bannon's place at Breitbart, and vice versa; Bannon introduced them to the site's founder, Andrew Breitbart, in 2011, and helped persuade them to invest $10 million in Breitbart's vision of an insurgent conservative media outlet that would take on Hollywood, the news media and established Washington figures, including conventional Republicans.
In exchange for their investment, the Mercers secured a seat for Bannon on Breitbart's board. When Breitbart died of a heart condition months later, Bannon took over the operation.
He then set about turning it into a clarion of economic populism and nationalist sentiment. It advocated for strict limits on immigration, particularly from Latin America and from Muslim-majority nations, and for an "America-first" agenda in trade negotiations.
Its political philosophy was amplified by Trump when he announced his candidacy in 2015, although Breitbart steered a relatively even line during the early primaries between Trump and Republican challenger Ted Cruz, the Mercers' preferred candidate.
Bannon at one point described Breitbart as "the platform for the alt-right," a phrase that became associated with white separatism, anti-Semitism and generally racist sentiments. Breitbart's editors insisted the site endorsed none of those views.
Nevertheless, Breitbart soared under Bannon, reaching 37 million unique readers a month before Trump upset Hillary Clinton to win the 2016 election.
Among the writers he championed was Milo Yiannopoulos, who elicited both a rapturous response from Breitbart's readers but heavy criticism elsewhere for columns about lesbians, blacks and Muslims.
