A prominent US television anchorman, Keith Olbermann, was suspended today for making donations to three Democratic candidates in this week's midterm elections, in breach of company rules.

US journalistic ethics are extremely strict, barring media employees from donating to political parties or any other political involvement that might cause a conflict of interest.

Although Rupert Murdoch contributed $2m (£1.23m) to the Republicans and commentators from his Fox News network also made donations, papers such as the New York Times, National Public Radio and television channels such as CNN and MSNBC seek to maintain strict neutrality.

Olbermann, host of Countdown with Keith Olbermann, made contributions to two Democrats standing in Arizona and the Democrat Jack Conway, fighting for a place in the US Senate in Kentucky against the Republican Rand Paul, a Tea Party favourite.

Olbermann confirmed in a statement to the Politico website that he had given each of the candidates $2,400, the legal maximum for donations.

The MSNBC president, 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." Olbermann has a contract with MSNBC until 2012.

MSNBC has positioned itself in recent years as a liberal counterweight to the rightwing Fox News, with journalists such as Olbermann outspoken in support of Obama and the Democrats. But it still wants to be seen as adhering to journalistic standards of objectivity, and has been criticising Murdoch over his donations.

Olbermann contributed to one of the Arizona candidates, Raúl Grijalva, on the same day that the Democrat appeared on his Countdown show.

In his statement, Olbermann said: "One week ago, on the night of Thursday 28 October 2010, after a discussion with a friend about the state of politics in Arizona, I donated $2,400 each to the re-election campaigns of Democratic representatives Raul Grijalva and Gabrielle Giffords. I also donated the same amount to the campaign of Democratic senatorial candidate Jack Conway in Kentucky."

He added: "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."

The Fox News commentator Sean Hannity, according to the Salon website, gave $5,000 to a political committee that supported Michele Bachmann, a Republican congresswoman and Tea Party favourite. But Fox, unlike MSNBC, sees no conflict of interest.

In a sign of the strict neutrality the US media normally applies, the former editor of the Washington Post, Len Downie, famously did not even vote. Another, more recent example was a ban by NPR on staff – other than those covering the event – attending Jon Stewart's rally on the Washington Mall last weekend.