As a proudly renegade voice of the populist right, Breitbart News has long delighted in bedeviling liberals and establishment Republicans alike, emerging in recent years as one of the nation’s leading conservative media outlets.

But in an ironic twist, Breitbart, a news and opinion website that welcomed the rise of Donald J. Trump as an outsider candidate, is now facing a problem similar to the one roiling the Republican Party it likes to torment: a scathing internal dispute, with Mr. Trump at its center.

Several top executives and journalists at the site have resigned in the last week, saying the organization has turned into a shill for the Trump campaign and failed to support Michelle Fields, a Breitbart reporter who accused Mr. Trump’s campaign manager, Corey Lewandowski, of grabbing and shoving her at a Florida rally last week.

Ms. Fields reported the episode to the police, and a journalist for The Washington Post, Ben Terris, identified Mr. Lewandowski as the person who grabbed her. The Trump campaign has denied Ms. Fields’s account, and Breitbart, after initially requesting an apology, later published an article casting doubt on whether Mr. Lewandowski was involved.