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Andrew Breitbart , a conservative blogger and activist who used undercover video to bring discredit and disgrace to his liberal targets, died Thursday. He was 43.

Mr. Breitbart was as polarizing a figure as he was popular. Hailed by the political right as a truth-teller who exposed bias and corruption, he was derided by many on the left as a provocateur who played fast and loose with the facts to further his agenda.

Among Mr. Breitbart’s biggest coups was forcing the resignation of a New York congressman, Anthony D. Weiner. Someone in Mr. Breitbart’s vast network of tipsters and fans e-mailed him sexually explicit photos that Mr. Weiner had taken of himself and sent to women online. Mr. Breitbart published the photos on his Web site, BigGovernment.com, igniting a political firestorm that reached the highest levels of Congress.

The move displayed Mr. Breitbart’s signature willingness to flout authority and his innate sense for creating a viral news story. He was already a cult figure in conservative media when the Weiner scandal cemented Mr. Breitbart’s status on the right as a force on par with Rush Limbaugh and Matt Drudge.

Mr. Breitbart’s techniques made him a pioneer for conservative media. What Mr. Limbaugh was to the radio and what Mr. Drudge was to the Internet, Mr. Breitbart was to online video and images.

The causes of Mr. Breitbart’s death were not immediately made public. Lt. Larry Dietz, watch commander for the Los Angeles County Coroner’s Office, confirmed Mr. Breitbart’s death on Thursday.

Ed Winter, a spokesman for the coroner’s office, said Mr. Breitbart was found “evidently collapsed” but would not say where. Paramedics responded and transported him to the U.C.L.A. Medical Center, which reported that Mr. Breitbart had died just after midnight.

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“He does have some medical history issues, but he hadn’t seen a doctor in the last year or so,” Mr. Winter said. He would not say exactly what those medical issues were, but said the coroner’s department was investigating and he would probably have more information later in the day.

Mr. Breitbart’s father-in-law told The Associated Press that he was walking near his home in Los Angeles when he collapsed.

The news spread on Twitter, where followers of the conservative blogger expressed shock.

“Absolutely stunned to hear about Andrew Breitbart,” Brad Dayspring, a spokesman for Eric Cantor, the majority leader in the House, wrote on Twitter. “Thoughts and prayers go out to his friends, family, and colleagues.”

Mr. Breitbart was one of the early — and most controversial — users of blogs to disseminate political information and rumors. Early in his career, Mr. Breitbart worked with Mr. Drudge, manning the Drudge Report during the hours when Mr. Drudge was off. Arianna Huffington gave Mr. Breitbart one of his first big breaks, hiring him as a researcher when her politics leaned more to the right. He also played a role in the creation of her Web site, The Huffington Post.

At the top of Drudge Web site, was the following note to readers:

In the first decade of the DRUDGEREPORT Andrew Breitbart was a constant source of energy, passion and commitment. We shared a love of headlines, a love of the news, an excitement about what’s happening. I don’t think there was a single day during that time when we did not flash each other or laugh with each other, or challenge each other. I still see him in my mind’s eye in Venice Beach, the sunny day I met him. He was in his mid 20’s. It was all there. He had a wonderful, loving family and we all feel great sadness for them today…

Ms. Huffington praised him for “his passion, his exuberance, his fearlessness.”

In 2009, Mr. Breitbart started the first in a series of “Big” blogs with names like “Big Journalism,” “Big Hollywood” and “Big Government.” The Web sites gave Mr. Breitbart a big online perch of his own and he quickly became a part of the conservative firmament, drawing hundreds of thousands of visitors a month.

The Acorn scandal of 2009 originated with Mr. Breitbart when a young conservative activist named James O’Keefe came to him with undercover video of Acorn workers apparently offering advice on how to evade taxes and conceal child prostitution. The videos went viral once Mr. Breitbart published them. In response, Congress ended grants to Acorn, and federal agencies severed ties with the group.

Mr. Breitbart earned a reputation for being playful but also selective with the facts; in an infamous case in 2010, he helped to instigate the firing of an Agriculture Department official, Shirley Sherrod, by publishing a misleading video clip of her speaking. At the time of his death, she was suing him for defamation.

In a statement quoted by The Associated Press, Ms. Sherrod said: “My prayers go out to Mr. Breitbart’s family as they cope during this very difficult time. I do not intend to make any further comments.”

Mr. Breitbart relentlessly attacked what he called liberal bias in the media, though he frequently consented to interview requests from the news outlets that he labeled that way. Once, when he was pitching a story idea to a reporter at The New York Times, he playfully wrote in the subject line of the e-mail, “Am I off-limits?” though he knew he was not.

He was vilified by liberals who believed that he coarsened the public discourse and often found ways to make himself the center of attention — as when he showed up at a news conference arranged by Representative Weiner last June and started taking questions from journalists.

The news conference was sparked by Mr. Breitbart’s publishing photos of a bare-chested Mr. Weiner on his site BigGovernment.com; the ensuing scandal over Mr. Weiner’s online contacts with women via Twitter and Facebook led to Mr. Weiner’s resignation days later.

But he was admired by many conservatives. Michelle Malkin, a conservative blogger and television commentator, wrote in a blog post Thursday that his legacy “online and in the conservative movement is built to last.” She called him “a mentor to the next generation of right-wing activists and citizen journalists.”

A reporter traveling with the Republican presidential candidate Rick Santorum said that Mr. Santorum was visibly shaken when asked about Mr. Breitbart’s death. He called the death a huge loss for the country and the conservative movement.

Speaking on the Fox News Channel, the conservative columnist Jonah Goldberg said that Mr. Breitbart “truly loved the fight.”

Noting that Mr. Breitbart would regularly post to Twitter the incendiary remarks that people would send him, Mr. Goldberg said “it was his Wheaties — that’s what he had for breakfast.”

Michael D. Shear, in Washington, and Ian Lovett, in Los Angeles, contributed reporting.