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.

The NPR “sting” showed two fundraisers for the media organization making disparaging comments about conservatives during a lunch in Georgetown. Among other things, the edited tape presented one of the NPR executive’s more incendiary comments as his own when, in fact, he had been quoting others. NPR’s chief executive, Vivian Schiller, resigned a day later as conservative lawmakers intensified efforts to deny federal funding to public broadcasters (that effort failed).

Lowry said Mr. Breitbart “wasn’t a journalist in the traditional sense of dotting every i and crossing every t.” As such, his journalistic lapses “didn’t damage him on the right. He wasn’t playing by the traditional media rules.”

Kindness from rivals

Mr. Breitbart’s political adversaries are speaking kindly about a man they often vilified.

“There’s no point in engaging in political debate today,” said Ari Rabin-Havt, executive vice president of Media Matters for America, a liberal media-criticism group that once branded Mr. Breitbart “an ideological liar.” “My sympathy is with his wife and children.”

Arianna Huffington, founder of the liberal-leaning Huffington Post, said in a statement: “I was asked many times this morning for my thoughts on what Andrew meant to the political world, but all I can think of at the moment is what Andrew meant to me as a friend, starting from when we worked together — his passion, his exuberance, his fearlessness.” Mr. Breit­bart helped Huffington start her Web site in 2005.

Mr. Breitbart had four young children, one of whom he named William Buckley after the late conservative icon. Mr. Breitbart’s wife, Susannah, is the daughter of actor Orson Bean.

Mr. Breitbart — who was of Irish American heritage but was adopted as a baby by a Jewish couple who raised him in the wealthy Brentwood section of Los Angeles — was an indifferent prep-school student who found his niche writing for his school paper. He first ran into controversy when he made up a quote and attributed it to a classmate.

After graduating from Tulane University in New Orleans, he spent several years as a waiter, a computer programmer and freelance writer for an alternative music magazine. In an interview with ABC News in 2010, he said the Clarence Thomas-Anita Hill hearings in 1991 were “the moment that I saw a glimpse of the matrix” — the supposed conspiracy of political liberals and media elites.

Striking out on his own

In the mid-1990s, as the Internet took hold, Mr. Breitbart met a fellow Los Angeleno, Matt Drudge, who had started the conservative Drudge Report, and went to work there as an editor.

While with Drudge, he met Huffington, then a conservative commentator, and left Drudge to do research for her. “I was a slacker” at the time, he told Time magazine. “Writing, rhetoric, argument — she demanded that I take a disciplined approach.”

They split soon after. “I’m happy Arianna has the Huffington Post,” he said later. “Let it be known that I seethe with contempt for 90 percent of what appears on that site.”

That kind of pugnacity served Mr. Breitbart well in building his own Internet empire.

Not long after leaving Huffington in mid-2005, he founded an eponymously named site, modeled on the Drudge Report but with an even more partisan tone. He began appearing frequently on Fox News shows and became a columnist for the Washington Times. Big Government began in the fall of 2009, making a huge splash with the ACORN video.

News of Mr. Breitbart’s death sparked such hostile responses from some readers that The Washington Post had to pre-moderate the comments section on its Web site. But it’s unlikely Mr. Breitbart would have minded. He was unafraid of critics.

“They think they can take me down, that they can hurt me,” he said in a 2010 interview with Slate. “It just makes me bigger.”