Stephen Bannon will leave Breitbart to take a senior position on Donald Trump’s campaign staff. (Kirk Irwin/Getty Images For Siriusxm)

Donald Trump’s selection of Breitbart Media chairman Stephen K. Bannon as his campaign’s new chief executive made official a role Bannon has played from afar for the past few years — that of ideological soul mate and cheerleader for the Republican nominee.

Bannon and Breitbart, a leading organ of the conservative media, threw their editorial energies behind Trump long before Trump declared he was running in 2015. On the satellite radio program he hosts and via the Breitbart website, Bannon has championed Trump’s agenda, growing so similar in tone that critics have dismissed Breitbart News as “Trumpbart” or “Trump Pravda.”

The site particularly is in sync with Trump’s anti-immigrant, anti­-Muslim rhetoric (sample head­line on Wednesday: “Pakistani Woman Accused of Forcing Daughter Into Prosti­ution To Finance Pilgrimage to Mecca”), and Trump’s enduring hostility toward the Republican Party’s “establishment,” most prominently Speaker of the House Paul D. Ryan.

In an interview this year, Bannon referred derisively to Ryan as being “grown in a petri dish at the Heritage Foundation,” the anti­thesis of the populist grass roots that Breitbart stokes and celebrates.

As such, Trump’s appointment of Bannon on Wednesday will probably intensify whatever hard feelings already exist between Trump and the GOP’s other leading figures during the campaign’s final months. It also seems more likely to help Trump among the “insurgent” wing of the party than among the independents and moderates that Trump will need to defeat Hillary Clinton in November.

The Fix's Chris Cillizza explains why Donald Trump demoted campaign chief Paul Manafort and added two new top advisers – Breitbart News chief Stephen Bannon and pollster Kellyanne Conway. (Peter Stevenson/The Washington Post)

The news that Trump had hired Bannon as chief executive and pollster Kellyanne Conway as his new campaign manager appeared to take Bannon’s underlings at Breitbart News by surprise. The website cited the New York Times in its first story about Bannon’s appointment.

Candidates and presidents have often been helped by like-minded journalists. President Obama named former Time magazine reporter Jay Carney as his press secretary in 2011; President George W. Bush ap­pointed a former Fox News host and newspaper columnist, Tony Snow, as one of his press secretaries.

But Bannon’s ascension might be something new: the first time a major presidential campaign will be run by the man who directed an entire media organization to become aligned with the can­didate before his appointment. Breitbart, which says it reaches about 31 million unique visitors per month, isn’t expected to divert from its pro-Trump line after Bannon temporarily vacates his position there.

Bannon, 62, has been a Navy officer, a documentary film­maker and an investment banker, first with Goldman Sachs and later with his own firm, which specialized in media and enter­tainment. (His company was an investor in Castle Rock Enter­tainment and wound up with a piece of the syndication revenue from reruns of the company’s biggest show, “Seinfeld.”)

Although his documentaries focused on conservative political figures, including Ronald Reagan, Sarah Palin and Andrew Breitbart himself, Bannon has never managed or officially advised a national political campaign before. In fact, he had little prominence or direct involvement in politics until he began his formal association with Andrew Breitbart in 2010.

Bannon offered Breitbart office space for his fledgling digital media company and stepped in as executive chairman in 2012 after Breitbart died of a heart attack. He has since become the face and voice of the company, hosting its morning radio program, appearing as a panelist on Fox News and formerly writing a column.

He did not respond to emails seeking comment on Wednesday.

Republican presidential nominee Donald Trump, who's spent months touting his ability to "win," has recently started to discuss the possibility of losing the election – a surprising move for a candidate. (Peter Stevenson/The Washington Post)

Breitbart News was among the first outlets to take a Trump campaign for president seriously, trailing him to speeches before small conservative groups in 2014. The most frequent byline on those early Trump stories was that of reporter Matthew Boyle, who has since become the site’s political editor. Boyle, who declined to comment Wednesday, enjoyed liberal access to Trump, interviewing him frequently at Trump Tower and occasionally on his jet.

Breitbart’s loyalty to Trump was such that Bannon and other senior executives took a skeptical view of one of their own reporters, Michelle Fields, after Fields complained that Trump’s then­campaign manager Corey Lewandowski had grabbed her and yanked her arm while she sought to speak to Trump after a news conference in March.

An eyewitness and later video of the incident clearly showed Lewandowski physically restraining Fields, but Breitbart’s reporting took Trump’s side by questioning Fields and a supportive colleague, columnist Ben Shapiro. One Breitbart story that trashed Shapiro used a fake byline — that of Shapiro’s father.

Fields and Shapiro, as well as three other Breitbart staffers, resigned over the incident, as did Breitbart’s public-relations counsel, Kurt Bardella. Fields is now a reporter for the Huffington Post and Shapiro is the co-founder of and a columnist at the Daily Wire.

In interviews, Bardella and Shapiro both disparaged Bannon as dishonest and imperious (Fields declined to speak).

“He’s a bully who has a very casual relationship with the truth,” said Bardella, who runs his own PR firm in Washington. “Trump seems to be running a campaign based on theater, propaganda and the absence of facts. That aligns perfectly with Steve Bannon.”

Shapiro said Bannon will likely win no matter what the outcome of the campaign. If Trump defeats Clinton, he said, Bannon will become White House chief of staff. If Trump loses, Shapiro predicted Bannon would recruit the billionaire businessman to back a new media venture with ousted Fox News chairman Roger Ailes and Fox News host Sean Hannity, a loyal Trump supporter.

“Steve is a very bright guy,” said Shapiro. “He’s also vicious and has no principles.” He added, “He’ll go far.”