LOS ANGELES >>  Conservative media publisher and activist Andrew Breitbart, who was behind investigations that led to the resignations of former Rep. Anthony Weiner of New York and former U.S. Agriculture Department official Shirley Sherrod, has died in Los Angeles. He was 43.

Breitbart’s website, bigjournalism.com, announced Thursday he died of natural causes in Los Angeles. His death was confirmed by breitbart.com editor-in-chief Joel Pollak, who said he was at the hospital, and by the Los Angeles County coroner’s office.

Breitbart was walking near his house in the Brentwood neighborhood on Thursday, shortly after midnight, when he collapsed, said his father-in-law, Orson Bean, a movie, TV and Broadway actor.

Someone saw Breitbart fall and called paramedics, who tried to revive him. They rushed him to the emergency room at Ronald Reagan UCLA Medical Center in Los Angeles.

Breitbart had suffered heart problems a year earlier, but Bean said he could not pinpoint what happened.

"I don’t know what to say. It’s devastating," Bean told The Associated Press.

Larry Dietz, watch commander at the Los Angeles County coroner’s office, said a cause of death was unknown and an autopsy was likely.

Reaction to his death was quick.

"RIP ‘O Mighty Warrior!" Texas Gov. Rick Perry said in a message on Twitter, the medium where Breitbart confronted his critics with often abrasive messages. Indeed, Breitbart’s final message called a follower "a putz."

His online profile, meanwhile, called him a "mild-mannered family guy" and "husky male model."

Media Matters, the liberal watchdog that was a frequent Breitbart critic, said the organization’s "thoughts and prayers are with his family today."

"We’ve disagreed more than we’ve found common ground, but there was never any question of Andrew’s passion for and commitment to what he believed," said Media Matters’ Ari Rabin-Havt.

Republican presidential contenders also weighed in.

Rick Santorum called Breitbart a "powerful force" after learning of his death from reporters at a rally in Dalton, Ga. "He will be what a huge loss … for our country and certainly for the conservative movement and my prayers go out to his family," Santorum told reporters. "I’m really sorry to hear it."

Mitt Romney posted to Twitter: "Ann and I are deeply saddened by the passing of (at)AndrewBreitbart: brilliant entrepreneur, fearless conservative, loving husband and father."

Newt Gingrich tweeted: "Andrew Breitbart was the most innovative pioneer in conservative activist social media in America. He had great courage and creativity."

Breitbart’s fans have praised him for exposing government corruption and media bias.

Sherrod, who is black, was fired from her job as Georgia state rural development director in July 2010 after an edited video surfaced of her making what appeared to be a racist remark. She is seen telling a local 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 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.

Sherrod sued Breitbart, his employee, Larry O’Connor, and an unnamed "John Doe" defendant for "defamation, false light and intentional infliction of emotional distress." A lawyer for O’Connor said Thursday that it is unclear whether the case will proceed for the other two defendants, who were in the process of trying to get the case dismissed from federal court.

Breitbart’s websites also featured a 2009 hidden-camera video that brought embarrassment to the community group Association of Community Organizations for Reform Now. The videos show ACORN staffers offering advice on taxes and other issues to actors posing as a prostitute and pimp.

Those videos triggered a firestorm of criticism, with some ACORN employees appearing willing to support illegal schemes involving tax advice, misuse of public funds and illegal trafficking in children.

An internal review later concluded the videos "feed the impression that ACORN believes it is above the law." The internal investigation and a Government Accountability Office report cleared ACORN of criminal activities.

Even so, public pressure led Congress to block previously approved funds from going to ACORN and to stop future payments. Roughly 10 percent of ACORN’s funds came from federal grants and the group eventually disbanded.

Breitbart also sparked a controversy that ultimately led to the resignation of Weiner, whose problems began on May 28 when Bretibart’s biggovernment.com posted a lewd photograph of an underwear-clad crotch and said it had been sent from Weiner’s Twitter account to a Seattle woman.

Initially, Weiner lied, saying his account had been hacked. But he pointedly did not report the incident to law enforcement — a step that could have led the way to charges of wrongdoing far more serious than mere sexting.

Additionally, his public denials were less than solid — particularly when he told an interviewer that he could not "say with certitude" that he wasn’t the man in the underwear photo.

Weiner’s spokesman said the photo was just "a distraction" and that the congressman "doesn’t know the person named by the hacker."

The congressman denied sending the photo and said he had retained an attorney and hired a private security company to figure out how someone could pull off such a prank.

But Weiner dropped that story line on June 6, offering a lengthy public confession at a Manhattan news conference, acknowledging to online activity involving at least six women.

Breitbart seldom showed restraint in his vitriol to his critics and seemed to relish in the negative attention his antics earned him.

After Sen. Edward M. Kennedy of Massachusetts died in 2009, Breitbart tweeted "Rest in Chappaquiddick" and called him "a special pile of human excrement." When critics questioned his tone, he tweeted they "missed my best ones!"

Breitbart is survived by his wife, Susannah Bean Breitbart, and four children.

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Associated Press writers Mary Clare Jalonick, Jack Gillum and Brett Blackledge in Washington, Jeff Wilson in Los Angeles and Ray Henry in Dalton contributed to this report.