NEW YORK — Following several stories questioning Bill O’Reilly’s past reporting, a liberal media watchdog has ordered its researchers to comb through years of the Fox News Channel host’s writings, radio and television shows and public appearances to find examples of inconsistencies.

O’Reilly is squarely in the crosshairs of Media Matters for America, an illustration of how the media is subject to the same political campaigns as politicians. Fox is standing behind O’Reilly, but the extent to which cable news’ most popular personality is damaged may depend on how many more stories come out.

“It’s a moving target,” said Bradley Beychok, Media Matters president, on Friday. “It’s too early to tell what the end of this is.”

The effort began a week ago, after Mother Jones magazine reported that O’Reilly claimed to be in a “war zone” while reporting on the Falklands War in Argentina more than 30 years ago for CBS News, when instead he covered an anti-government demonstration more than 1,000 miles away from the front.

Other reports soon followed:

—Media Matters quoted police reports, witnesses and former O’Reilly co-workers to rebut O’Reilly’s assertion that he was outside the home of John F. Kennedy assassin Lee Harvey Oswald’s friend when the friend killed himself by shotgun.

—Media Matters said O’Reilly suggested he witnessed the assassination of four nuns in El Salvador when he had not.

—The Guardian quoted former O’Reilly colleagues who said he had exaggerated an incident where he said he’d been attacked by protesters during the 1992 Los Angeles riots. Instead, they said he had argued with one man.

Media Matters, an organization formed 11 years ago by David Brock, has an $11.2 million operating budget and a mission devoted to combatting conservative media. It has 45 researchers on staff and, as first outlined in Politico, assigned most of them to O’Reilly’s dossier after the Mother Jones article hit.

“This is not helpful to his brand,” Beychok said. “His brand is that he’s a truth-teller and we’ve had numerous examples of where he’s not telling the truth.”

Given O’Reilly’s prominence and Media Matters’ mission, it’s somewhat surprising the group hadn’t dug into O’Reilly’s record like this before. Beychok said the Media Matters is a “rapid response research organization” that follows the news.

Media Matters hasn’t actively followed the case of NBC’s Brian Williams, suspended for misrepresenting his experiences while reporting in Iraq in 2003, because it isn’t central to its political mission. On the other side, the conservative Media Research Center, which said Williams had gotten off easy with a six-month suspension, did not respond to questions about O’Reilly on Friday.

O’Reilly’s publishers have backed their author, reiterating that point on Friday. O’Reilly is the co-author of a successful book series on the deaths of prominent historical figures like Kennedy, Abraham Lincoln and Jesus Christ.

“This is nothing more than an orchestrated campaign by far left advocates Mother Jones and Media Matters,” Fox spokeswoman Irena Briganti said. “Responding to the unproven accusation du jour has become an exercise in futility. Fox News maintains its staunch support of O’Reilly, who is no stranger to calculated onslaughts.”

For his part, O’Reilly has reacted differently to the stories. He immediately went on the attack after the Mother Jones article, calling it a “political hit job” in several interviews and saying that the violence in the Buenos Aires demonstration he covered justified his description that it was a combat zone. He explained his statement about the nuns’ assassinations by saying he saw pictures of the aftermath. He hasn’t responded to the stories about the Kennedy figure or Los Angeles riots.

The only way O’Reilly can be seriously damaged is if more allegations about his statements come forward from sources other than partisan organizations, said Joe Muto, a former O’Reilly producer fired by Fox after he began writing an anonymous blog as the “Fox mole.”

“Ultimately, he’ll survive this because he’s not held — by his bosses, or the public, or himself — to the same standards of truth-telling as Brian Williams is,” Muto said. “People expect a certain degree of hyperbole and exaggeration from O’Reilly. It’s baked into the job description. It’s part of his persona.”

His attackers are the liberal organizations that O’Reilly and his fans find perfect foils, he said.

“The only way that list of nemeses could be more perfect for him is if it included Al Franken, Ward Churchill and the entire state of Vermont,” said Muto, who now edits for reality television programs. “Bill is never happier than when he’s under attack.”