Reverend Al Sharpton speaks to residents attending a protest rally demanding justice for the killing of black teenager Trayvon Martin in Miami, Florida April 1, 2012. (Lucas Jackson / Reuters/Reuters)

On his MSNBC program on Tuesday, Al Sharpton told viewers about the rallies being planned to protest George Zimmerman’s acquittal on murder and manslaughter charges in the killing of Florida teenager Trayvon Martin. “I’ve said from the beginning we must pursue [this] until the end,” declared Sharpton, adding, “We’ll be in 100 cities on Saturday.”

Sharpton certainly knew whereof he spoke. The “Justice for Trayvon” rallies, after all, are being organized by Sharpton himself through the organization he heads, the National Action Network.

Earlier in the day, Sharpton led a group of ministers to the doors of the Justice Department in Washington to demand that Zimmerman be charged with violating Martin’s civil rights. The story got wide coverage online and on TV.

And so, in just a few hours, Sharpton, 58, played several parts in the Martin story virtually at once: national TV host, Martin-family advocate, rally organizer and promoter, and newsmaker.

The multiple roles, which Sharpton has taken on since the Martin-Zimmerman story’s earliest days, make him an unusual figure among TV news personalities. Perhaps only Karl Rove, the prominent Republican Party fundraiser and Fox News Channel pundit, comes close to being such an active participant in the news stories he goes on TV to talk about.

But even in an age of rapidly eroding boundaries between reporters and commentators, Sharpton’s multi­tasking stands out. A veteran champion of issues involving African Americans — from the discredited claims of Tawana Brawley to the vindication of Amadou Diallo — Sharpton helped draw national attention to Martin’s shooting last year by leading a rally in Sanford, Fla., to demand Zimmerman’s arrest. He has helped raise money for the Martin family. And he has used his nightly TV show, “Politics­Nation,” as a forum to advocate on their behalf.

Sharpton’s immersion in the story — unthinkable for a network-news figure even a few years ago — has raised questions for MSNBC and its parent, NBC News. Among them: Is Sharpton, and MSNBC, helping to create some of the very news MSNBC is covering?

MSNBC’s president, Phil Griffin, acknowledged in an interview that Sharpton is different from the network’s other hosts; indeed, Griffin hired him in 2011 with a “carve out” from NBC News’ policy of prohibiting employees from direct involvement in political activity.

But the decision was worthwhile, he said: “We didn’t hire him to be just another news host. I knew who we were hiring. He brings to our channel a different voice, and a voice who speaks about issues that are not being talked about regularly anywhere else. . . . I think having Rev. Sharpton on our air is a major plus for this network.”

He adds that MSNBC has been “transparent” with viewers about Sharpton’s activities off the air. The only major restriction MSNBC appears to have placed on Sharpton is fundraising for the Martin family. Sharpton’s effort last year was a “one-time event,” Griffin said. “We talked about it,” and Sharpton hasn’t repeated it since.

Sharpton, who declined to comment for this report through an MSNBC representative, addressed the issue of TV punditry and activism when he began his MSNBC program. In an 2011 interview with the Tampa Bay Times, he noted that he has engaged in similar work without issue in his radio show, “Keeping It Real with Al Sharpton,” for more than five years.

He also said fellow MSNBC hosts Rachel Maddow and Ed Schultz “do what I do . . . a type of journalism based on opinion and advocacy,” although neither Schultz nor Maddow led advocacy groups before or after becoming MSNBC employees.

Still, Sharpton’s centrality to the Martin story “will make an interesting case study for a journalism ethics class,” said Lucy Dalglish, dean of the University of Maryland’s college of journalism. “Certainly, an activist has every right to communicate [his opinions], whether it’s on TV, or their own Web site,” she said. “Perspective in covering the news is one thing, but when it moves from perspective to activism and you’re actually one of the people driving the story you’re covering — that’s a little bit strange. There have got to be some people at NBC who are very troubled by this. Or there should be.”

MSNBC’s acceptance of Sharpton’s advocacy role stands in stark contrast to its rival Fox News, which takes a dimmer view of extracurricular activity by its hosts. In 2010, for example, the network canceled host Sean Hannity’s broadcast from a tea party rally because, it said, the rally’s organizers stood to profit from Hannity’s presence.

Sharpton’s involvement in the Martin story is striking, too, in light of MSNBC’s own actions against its morning host, Joe Scarborough, and former host Keith Olbermann in late 2010. Both men were briefly suspended after the disclosure that they had made a series of small political donations.

While that appears to be a more passive role than Sharpton has played, Griffin drew a distinction. He said Sharpton’s political activities were known to both management and viewers, whereas Scarborough and Olbermann failed to disclose their contributions in violation of NBC News policy prohibiting employees from making donations without approval. “We talk through everything he does,” Griffin said. “It’s always upfront and clear to viewers what his role is.”

But Sharpton’s involvement with Martin is so extensive that full disclosure isn’t always made. During an appearance this week on NBC’s “Today,” Sharpton said that “ministers” were organizing Saturday’s rally, ignoring his own role.

Despite his close association with the Martin-Zimmerman story, Sharpton hasn’t been able to close the ratings gap between his program and Fox News’ “Special Report” with anchor Bret Baier. “PoliticsNation” averaged 732,000 viewers in its first four broadcasts since the jury’s verdict, compared with 1.89 million for Fox (CNN’s “Situation Room” was a distant third, with an average of 422,000 viewers).

Nevertheless, Sharpton gets a thumbs up from the National Association of Black Journalists. “Rev. Sharpton has never claimed to be a journalist, so therefore, as to the question of the ethics of his participation in protests and rallies surrounding the Trayvon Martin tragedy, I’m not sure that the same rules apply as it would to, say, a reporter or anchor,” said Gregory Lee Jr., the organization’s president.

“I said at the time of the Rev.’s hiring that I am pleased that he represents a growing amount of on-air diversity at cable networks,” Lee said, adding: “It is of the utmost importance that the nation’s television networks, radio stations, newspapers, magazines and online outlets represent the diversity of our viewers, listeners and readers.”