Donald J. Trump has shaken up his presidential campaign for the second time in two months, hiring a top executive from the conservative website Breitbart News and promoting a senior adviser in an effort to right his faltering campaign.

Stephen Bannon, the executive chairman of Breitbart News LLC, will become the Republican campaign’s chief executive, and Kellyanne Conway, a senior adviser and pollster for Mr. Trump and his running mate, Gov. Mike Pence of Indiana, will become the campaign manager.

Paul Manafort, the campaign chairman, will retain his title. But the staffing change, hammered out on Sunday and set to be formally announced Wednesday morning, was seen by some as a demotion for Mr. Manafort.

The news, first reported by The Wall Street Journal, was confirmed early Wednesday by Ms. Conway in a brief interview, but she rejected the idea that the changes amounted to a shake-up and said that Mr. Manafort was not being diminished.