AP

Andrew Breitbart, the conservative media entrepreneur and activist who published photos that led to Rep. Anthony Weiner's resignation from Congress last year, died unexpectedly early Thursday after collapsing while on a walk near his house in southern California. He was 43.

A message posted on Breitbart's websites on Thursday read, "We have lost a husband, a father, a son, a brother, a dear friend, a patriot and a happy warrior. Andrew lived boldly, so that we more timid souls would dare to live freely and fully, and fight for the fragile liberty he showed us how to love."

Breitbart died just after midnight at Ronald Reagan UCLA Medical Center, Larry Dietz, a spokesman for the L.A. county coroner's office, told CBS News. Breitbart's father-in-law Orson Bean told the Associated Press that Breitbart was walking near his house in the Brentwood neighborhood when he collapsed.

Breitbart, an online publisher who ran multiple sites including Breitbart.com and BigGovernment.com, was known for breaking news and creating narratives that stirred controversy for Democrats. In addition to publishing the photos of Weiner last year, Breitbart also published an out-of-context video from 2010 that created a controversy surrounding then-U.S. Agriculture Department employee Shirley Sherrod.

Breitbart is survived by his wife Susannah Bean Breitbart, 41, and four children.

Conservatives Thursday morning mourned Breitbart's death. Upon learning the news, Republican presidential candidate Rick Santorum called it "shocking."

"What a powerful force," he said of Breitbart. "It's almost - you think of anybody out there who's just got more energy, and just, out there constantly, driving and pushing. What a huge loss, in my opinion, for our country, and certainly for the conservative movement."

Via Twitter, Mitt Romney said, "Ann and I are deeply saddened by the passing of @AndrewBreitbart: brilliant entrepreneur, fearless conservative, loving husband and father."

Erick Erickson of the conservative blog RedState.com said Breitbart's death was "like a supernova has gone dark."