Mr. Breitbart used his website to promote a hidden-camera video with actors posing as customers that led to the downfall of the liberal Association of Community Organizations for Reform Now, or ACORN. He posted explicit photos of then-Rep. Anthony Weiner that led the New York Democrat to resign in a sexting scandal, and an edited video that led former U.S. Agriculture Department official Shirley Sherrod to resign over since-reversed perceptions she was a racist.
In a new media age, Mr. Breitbart argued that anyone with a laptop could reshape public discourse. He used his skills at sites such as Big Journalism and Big Government, and his takedown of Weiner established him as a conservative media star.
His business partner and lifelong friend, Larry Solov, once said Mr. Breitbart had two speeds: lighthearted jokester and fiery culture warrior. "They flip back and forth," Solov said. "And there is not that much in between."
Mr. Breitbart died after collapsing shortly after midnight during a walk near his home. He had suffered heart problems a year earlier, but his father-in-law, actor Orson Bean, said he could not pinpoint what happened.
Larry Dietz, watch commander at the Los Angeles County coroner's office, said an autopsy was likely.
Mr. Breitbart leaves behind his showcase, a family of websites that waged daily war with what he considered liberal bias in the media, on college campuses, and in the entertainment industry. Joel Pollak, an editor, said that Mr. Breitbart planned to launch a retooled version and that those plans would go forward.
"The core of what Andrew did was bring new citizen journalists into the new media," Pollak said. It "was, and still is, what we do."
It wasn't immediately clear who would take over the company, which once ran out of Mr. Breitbart's basement and now employs about a dozen people. His anchor site, Breitbart.com, was visited by 1.7 million people in January, according to the website tracker comScore Inc.
Republican candidates for president were quick to offer praise and condolences after learning of Mr. Breitbart's death. Newt Gingrich tweeted: "Andrew Breitbart was the most innovative pioneer in conservative activist social media in America. He had great courage and creativity."
Mr. Breitbart played by his own standards. He faulted what he called the mainstream media for all manner of shoddy work and bias, but his aim could go off course, too.
Sherrod, who is black, was ousted as the USDA's state rural development director for Georgia in 2010 after an edited video surfaced of her making what appeared to be a racist remark. She was seen telling an NAACP group that she was initially reluctant to help a white farmer save his farm more than two decades ago, long before she worked for the USDA.
Missing from the clip was the rest of the speech, which was meant as a lesson in racial healing. Sherrod told the crowd she eventually realized her mistake and helped the farmer save his farm.
Once the entire video surfaced, Sherrod received numerous apologies from the administration, including President Obama. But she later sued Mr. Breitbart, his employee Larry O'Connor, and an unnamed third defendant for defamation. A lawyer for O'Connor said Thursday that it was not clear whether the case would proceed against the other defendants, who were seeking to dismiss the case in federal court.
