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Keith Olbermann, the pre-eminent liberal voice on American television, was suspended Friday after his employer, MSNBC, discovered that he made campaign contributions to three Democrats last month.

The indefinite suspension was a stark display of the clash between objectivity and opinion in television journalism. While Mr. Olbermann is anchor of what is essentially the “Democratic Nightly News,” the decision affirmed that he was being held to the same standards as other employees of MSNBC and its parent, NBC News, both of which answer to NBC Universal. Most journalistic outfits discourage or outright prohibit campaign contributions by employees.

Many prominent liberals and conservatives immediately called on MSNBC to reinstate Mr. Olbermann, who is usually outspoken but who had no comment on his suspension Friday. His agent did not respond to a request for comment.

As of Friday evening, tens of thousands of people had signed a petition titled “Put Olbermann Back On The Air NOW!” that was circulated by the Progressive Change Campaign Committee.

Mr. Olbermann’s contributions came to light in an article by Politico Friday morning. Mr. Olbermann acknowledged in a statement that last month he donated $2,400 to the campaigns of Representatives Raul Grijalva and Gabrielle Giffords of Arizona and Attorney General Jack Conway of Kentucky, who lost his Senate race to Rand Paul.

Related Questions for Keith Olbermann On the eve of his suspension from MSNBC, the anchor talked about his partisanship. Read more»

Mr. Olbermann also said, “I did not privately or publicly encourage anyone else to donate to these campaigns, nor to any others in this election or any previous ones, nor have I previously donated to any political campaign at any level.”

Several hours later, the president of MSNBC, Phil Griffin, said in a statement, “I became aware of Keith’s political contributions late last night. Mindful of NBC News policy and standards, I have suspended him indefinitely without pay.”

Mr. Griffin consulted with both Steve Capus, the president of NBC News, and Jeff Zucker, the chief executive of NBC Universal before making the decision to suspend Mr. Olbermann.

No one at NBC News, MSNBC’s parent, would speculate about what this might mean for Mr. Olbermann’s future, though two NBC executives privately suggested this was not a step toward firing him.

One executive said the network decided it was imperative to take this kind of strong action as a way of underscoring that MSNBC, while featuring prime-time shows that overtly support Democratic policy, remains a channel that adheres to fundamental journalistic values.

An executive with another television news organization, who asked not to be identified in offering analysis of competitors, said NBC may even see the disciplining of Mr. Olbermann as an opportunity to distinguish itself from Fox News, which has been increasingly criticized for taking Republican positions.

Fox’s parent company, News Corporation, came under fire from Mr. Olbermann and others for donating $1.25 million to Republicans earlier this year.

Separately, one of the top hosts on Fox, Sean Hannity, contributed to two Republican Congressional candidates earlier this year. Mr. Hannity made a $2,400 contribution to a Republican congressional candidate, John Gomez, a long-time friend of his, and a $5,000 contribution to the political action committee of Representative Michele Bachmann, Republican of Minnesota.

Mr. Olbermann’s program, “Countdown,” is the most popular hour on MSNBC, with about 1.1 million viewers a night. Years ago Mr. Olbermann gave voice to dissenting views about the Iraq war and about Bush administration policies when few others on television would, and more recently he helped advance the Obama administration’s push for a health care overhaul. Mr. Griffin has called the seven-year-old “Countdown” the tent-pole of MSNBC, which has rebuilt itself as a home for liberal-minded Americans. It recently adopted a new slogan, “Lean Forward,” which winks at its progressive bent.

Mr. Olbermann has long been a volatile figure inside MSNBC. He has fought repeatedly in the past with Mr. Griffin and other managers over editorial decisions and has been harshly critical of his former bosses.

There was some head-scratching about MSNBC’s decision, since it is well known that Mr. Olbermann is a liberal newsman. There were defenders, including a writer for the libertarian magazine Reason, Michael C. Moynihan, who wondered why MSNBC had a “one-size-fits-all policy” about contributions.

Mr. Moynihan asked, “Isn’t it unfair to hold Olbermann, who is one of the most partisan people on television (if not of Earth), to the same standards as, say, Brian Williams? Countdown exists to promote Democratic candidates and liberal policies, which is just fine by me. So why shouldn’t Olbermann, as a private citizen, be allowed to donate money to those candidates he plumps for on television?”

Many MSNBC employees were stunned by the suspension. Bloggers

seized on the fact that another MSNBC host, Joe Scarborough, had not

been reprimanded when he made a contribution to a Republican in 2006

(although an NBC spokesman said he had asked for permission).

Most television news organizations have rules about journalists contributing to political campaigns, but some make distinctions between their anchors and reporters and those offering opinions. For example, in the aftermath of the Olbermann suspension, CNN said its policy “prohibits full-time employees from making contributions to political parties or candidates,” but a CNN executive acknowledged that these rules did not cover part-time employees or political contributors.

News Corporation policy states that Fox employees should not make donations in the company’s name but that “this is not intended to discourage employees from making personal contributions to candidates of the political parties of their choice.”

MSNBC’s policy, as published by msnbc.com in 2007, states that “anyone working for NBC News who takes part in civic or other outside activities may find that these activities jeopardize his or her standing as an impartial journalist because they may create the appearance of a conflict of interest. Such activities may include participation in or contributions to political campaigns or groups that espouse controversial positions. You should report any such potential conflicts in advance to, and obtain prior approval of, the President of NBC News or his designee.”

There was no request for an exception from Mr. Olbermann in this case, according to NBC News executives.

One of the NBC executives agreed with the point, suggested by senior executives at other television news divisions, that Mr. Olbermann’s transgression in making the donation was compounded by the fact that he had anchored the election coverage on MSNBC, and had not just worked as a commentator on the results.

The suspension comes at a particularly tricky time for MSNBC. NBC Universal is in the midst of a change of ownership, with its potential new owner, Comcast, seeking final federal approval for the sale. It hopes to conclude the deal by the end of the year.

Bernie Sanders, the independent senator from Vermont, called Mr. Olbermann’s suspension “outrageous” in a statement, adding, “This is a real threat to political discourse in America and will have a chilling impact on every commentator for MSNBC.”

MSNBC initially said that Chris Hayes, the Washington editor of The Nation magazine and an MSNBC contributor, would fill in for Mr. Olbermann. But Mr. Hayes said on Twitter that he did not feel comfortable doing so under the circumstances, and the channel ended up having a straightforward news anchor, Thomas Roberts, fill in on Friday night.