The onward march of Fox News, the relentlessly rightwing channel that has revolutionised American television news by making it overtly partisan, has been boosted by an opinion poll that suggests it is the most trusted news operation in the country.

Almost half of all Americans surveyed in the poll of 1,151 registered voters said they trusted Fox News. That is a notably larger vote of confidence than the 39% who said they trusted Fox's great rival CNN, and vastly more than the credibility ratings of the traditional news networks ABC News (31%), CBS News (32%) and NBC News (35%).

The poll findings are vindication of the commercial strategy of Fox News, which is owned by Rupert Murdoch and is the brainchild of his controversial henchman Roger Ailes. Fox News threw out the old model of television news – pitched towards a mass audience across the political spectrum and aspiring to standards of fairness in reporting – and replaced it with an aggressive drive for a niche audience of rightwing voters.

The motto of the network is "Fair & Balanced", but in many respects it is anything but. "Fox News is not really a news network, it's a commentary network. Its news output is a small island in a vast sea of very conservative commentary," said Mike Hoyt, editor of the Columbia Journalism Review whose March issue features a cover story exploring the Fox News phenomenon.

The poll findings underline the partisan nature of both the network and its audience. When the respondents are broken down by party affiliation, an overwhelming 74% of Republican-leaning Americans trust the network, but only 30% of Democrats.

Dean Denham, president of Public Policy Polling, the North Carolina-based survey firm that carried out the poll, said the Fox strategy had been brilliant commercially but its implications were troubling. "That people see the network as trustworthy is worrying in terms of the future of reasoned debate in America. A lie screamed loudly will trump a truth ­spoken quietly," he said.

Denham said he had first-hand experience of Fox News's value judgments.

His firm had conducted a poll that produced some strikingly poor results for the Democrats in terms of their popularity ratings. At exactly the same time Fox News commissioned its own internal poll, which came up with more favourable results for the Democrats, yet the network decided to go with PPP's results rather than its own.

"That showed me that when they have the opportunity to go with something more negative about the Democrats, they will."

Qualms about the biased approach of the network has even reached the Murdoch family. Earlier this month the PR executive Matthew Freud, who is married to Rupert Murdoch's daughter Elisabeth, told the New York Times he was "ashamed and sickened by Roger Ailes's horrendous and sustained disregard of journalistic standards".

But for the time being such anxieties are unlikely to change the direction and strategy of Fox News, which is enjoying growing viewer figures and a mounting political influence through the groundswell of rightwing popular discontent displayed by the tea party movement.

The latest rankings by Nielsen Media Research for all cable networks at primetime during the period dominated by the Haiti earthquake and the bombshell election of a little-known Republican to the Massachusetts Senate seat put Fox News top of the list with an average of 3.2 million viewers. That put it above lifestyle and entertainment channels and well above CNN in 22nd place.

As further evidence of its pre-eminence, Glenn Beck, the network's most strident and emotive of rightwing hosts, was this week voted second favourite TV personality in the annual Harris Poll, behind only Oprah Winfrey.