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.

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 Breitbart’s biggest coups was forcing the resignation of a New York congressman, Anthony Weiner. Someone in Breitbart’s network of tipsters and fans emailed him sexually explicit photos that Weiner had taken of himself and sent to women online. Breitbart published the photos on his website, BigGovernment .com, igniting a political crisis that reached the highest levels of Congress.

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

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

The causes of Breitbart’s death were not immediately made public.

Lt. Larry Dietz, watch commander for the Los Angeles County Coroner’s Office, confirmed Breitbart’s death Thursday.

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

“He does have some medical history issues, but he hadn’t seen a doctor in the last year or so,” Winter said.

He would not say exactly what those medical issues were but said the coroner’s department was investigating.

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.

Breitbart had become one of the early — and most controversial — users of blogs to disseminate political information and rumors. Early in his career, Breitbart worked with Drudge, manning the Drudge Report during the hours when Drudge was off. Arianna Huffington gave 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 website, The Huffington Post.

In 2009, Breitbart started the first in a series of “Big” blogs with names like “Big Journalism,” “Big Hollywood” and “Big Government.” The websites gave 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 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 Breitbart published them. In response, Congress ended grants to Acorn, and federal agencies severed ties with the group.

Breitbart was vilified by liberals who believed that he coarsened the public discourse, 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.”