Andrew Breitbart, a conservative media pioneer who died unexpectedly early Thursday, had a knack for showmanship and an appetite for a fight. A digital-age provocateur who didn’t own a magazine or have a talk show, Mr. Breitbart nonetheless managed to dominate the political conversation by combining the tools of new media with old-fashioned pugnaciousness. Mr. Breitbart, 43, was rushed to UCLA Medical Center in Los Angeles after collapsing while walking near his home. Doctors were unable to revive him. A cause of death has not been announced. Over the past six years, Mr. Breitbart created Web sites that mimicked the Internet savvy of the Drudge Report and the Huffington Post, using them to promote conservative causes in a media world he saw as dominated by the left. When news outlets reported claims that tea party protesters had shouted racial epithets at an African American lawmaker who supported President Obama’s health-care legislation in 2010, Mr. Breitbart did more than dispute the accounts. He reviewed all available footage of the event. He found no slurs — and offered $10,000 to anyone who could show otherwise. He got no takers. “He was fearless, gutty and innovative,” said Rich Lowry, the editor of National Review, a conservative journal founded by William F. Buckley Jr. Lowry compared Mr. Breitbart to Abbie Hoffman, the “yippie” activist of the 1960s who used humor and political stunts to win media attention for his fight against the Vietnam War. Mr. Breitbart’s Web sites — Big Government, Big Hollywood, Big Journalism — were launchpads for videos that sparked controversy around the heretofore little-known community services group ACORN and an Agriculture Department official named Shirley Sherrod. He also helped promote a “sting” video of fundraisers for NPR making critical comments about Republicans, and he broke the news last year of the salacious online activities of Rep. Anthony Weiner (D-N.Y.) that led to Weiner’s resignation. Mr. Breitbart inspired outrage from those who considered his headline-grabbing behavior manipulative and deceitful. He exploited a flaw of the contemporary news media: That in the rush to attract online traffic, news sources sometimes cast aside context and basic fact-checking. Many outlets reported Mr. Breitbart’s “revelations” only to discover that the story was not as it had initially seemed. News organizations found, for example, that the ACORN videos — which purported to show a young Mr. Breitbart ally named James O’Keefe posing as a pimp seeking advice from the organization about how to establish a brothel and evade taxes — actually presented a heavily edited account of what had happened. An investigation by the California attorney general’s office concluded that O’Keefe had added footage of himself and an ally dressed in flamboyant costumes that they had not worn to ACORN’s offices. By then, it was too late for ACORN: Outraged conservatives in Congress stripped the organization of federal funding. The Sherrod and NPR videos also misrepresented key parts of those stories. In the former, Mr. Breitbart presented only a brief clip of a speech in which Sherrod acknowledged that she had discriminated against a white farmer when she was a state agriculture official in Georgia. The subsequent uproar in 2010 prompted the Obama administration to seek her resignation. By day’s end, however, the full video of Sherrod’s speech emerged, revealing that she had gone on to condemn her own actions and to seek justice for the white farmer. An embarrassed Agriculture Secretary Tom Vilsack apologized and offered Sherrod her job back.