Olbermann’s banishment leaves uncertain the immediate future of MSNBC’s top-rated show, “Countdown With Keith Olbermann.” MSNBC initially said Christopher Hayes, Washington editor of the Nation, would fill in for Olbermann Friday night — then revised the announcement to say that daytime anchor Thomas Roberts would host. Who will take the helm in coming days has not been decided, a spokesman said.

Olbermann’s audience at 8 p.m. weeknights averaged about 1.1 million people in October, according to the network. He launched “Countdown” in 2003.

Olbermann anchored MSNBC’s election coverage Tuesday night. The revelation of Olbermann’s contributions — first reported by Politico — means that the anchor was leading on-air coverage of races in which he had privately picked favorites.

Federal Election Commission records show that on Oct. 28 Olbermann gave $2,400 each to Arizona Reps. Raul Grijalva and Gabrielle Giffords, and on Oct. 29 he donated $2,400 to Kentucky Senate candidate Jack Conway. The donation to Grijalva came on the same day the Democratic incumbent appeared on “Countdown,” Politico reported. Conway and Giffords have also appeared on the show.

Under FEC rules, $2,400 is the most an individual donor can give to a candidate in a general-election campaign.

Conway lost to tea party favorite Rand Paul. Grijalva was declared the winner of his race late Thursday, and Giffords’s race remained too close to call as of Friday evening.

MSNBC President Phil Griffin said in a two-sentence statement that he learned of Olbermann’s campaign contributions late Thursday night, and “mindful of NBC News policies and standards,” he acted quickly. However, the language of NBC’s policy leaves open the possibility that Olbermann would have been in compliance had he sought permission from his superiors first — though they could have denied his request.

The policy cautions the network’s journalists against taking part in activities that might create an appearance of conflict of interest. They include “contribution to political campaigns, political action committees 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,” the policy states.

Media ethicists said Friday it is prudent for all journalists — even those paid to espouse opinions — to demonstrate their independence by not contributing to candidates.

“There’s a presumption that an opinion journalist is still his or her own agent, that they are not really on a team, that they speak for themselves and are not actually writing speeches or functioning as activists,” said Tom Rosenstiel, director of the Pew Research Center’s Project for Excellence in Journalism. “I don’t see a lot of good that comes from giving the money, and I do see some potential downsides, even if you think opinion journalists are different.”

But the lines can be blurred, especially on competing cable networks, where sharply etched partisan opinion is popular with viewers, and different networks have different standards. FEC records show that Sean Hannity of Fox News Channel gave a total of $9,800 this year — $5,000 to a political action committee called Many Individual Conservatives Helping Elect Leaders Everywhere, and $4,800 to New York GOP congressional candidate John Gomez in the primary and general elections.

Hannity’s role at Fox is somewhat different from Olbermann’s. He does not anchor news coverage. In any case, according to Fox parent News Corp.’s rules, employees generally can make “personal contributions to candidates of their choice.”

Olbermann said in a statement released Friday by MSNBC that he decided to donate to the two Arizona candidates after a discussion of state politics with a friend, and he followed up with the donation to Conway.

“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,” he said.

Exposés produced by fellow journalists in recent years have identified scores of journalists who have given to political campaigns, often in violation of their employers’ guidelines. (Journalists at The Washington Post are expected to refrain from partisan activity that could compromise the “ability to report and edit fairly,” according to policy.) MSNBC.com itself in 2007 reported on 143 journalists who made contributions from 2004 to 2008.