Donald J. Trump named as his new campaign chief on Wednesday a conservative media provocateur whose news organization regularly attacks the Republican Party establishment, savages Hillary Clinton and encourages Mr. Trump’s most pugilistic instincts.

Mr. Trump’s decision to make Stephen K. Bannon, chairman of the Breitbart News website, his campaign’s chief executive was a defiant rejection of efforts by longtime Republican hands to wean him from the bombast and racially charged speech that helped propel him to the nomination but now threaten his candidacy by alienating the moderate voters who typically decide the presidency.

It also formally completed a merger between the most strident elements of the conservative news media and Mr. Trump’s campaign, which was incubated and fostered in their boisterous coverage of his rise.

Mr. Bannon was appointed a day after the recently ousted Fox News chairman, Roger Ailes, emerged in an advisory role with Mr. Trump. It was not lost on Republicans in Washington that two news executives whose outlets had fueled the anti-establishment rebellion that bedeviled congressional leaders and set the stage for Mr. Trump’s nomination were now directly guiding the party’s presidential message and strategy.