Rupert Murdoch's News Corporation has doubled its backing of the Republican party in advance of next month's mid-term elections by awarding $1m to an interest group that is leading the attack on Democratic candidates through political TV advertising.

The donation, revealed first by the website Politico, was made to the United States Chamber of Commerce, which has a stated intention to spend $75m on November's elections. Most of its money has gone into "attack adverts" targeted against Democratic candidates.

This is the second downpayment in three months by Murdoch to the Republican cause in its battle to give Barack Obama a bloody nose in the 2 November elections to the Senate and House of Representatives. In June, News Corporation gave $1m to the Republican Governors Association.

That donation led to widespread criticism that Murdoch was blatantly in breach of journalistic ethics. His Fox News channel claims to be "fair and balanced" in its reporting of political races, yet when the association of Democratic governors called on the TV channel to add a disclaimer to its coverage, revealing that its proprietor was funding the Republicans, it made no such concession.

The latest donation is likely to rekindle the debate about Murdoch's influence on the American political process. Fox News has frequently come under scrutiny for the unashamedly slanted nature of much of its coverage, particularly through commentators such as the Tea Party-backing Glenn Beck.

The dispute has reached as far as the White House. Obama told Rolling Stone magazine this week that in his opinion Fox News had "a point of view that I think is ultimately destructive for the long-term growth of a country that has a vibrant middle class and is competitive in the world".

News Corporation has consistently denied a political motive in its sponsorship of interest groups. It claims instead to be backing a "pro-business" stance.

But a survey by the Wesleyan Media Project shows that of interest groups buying political advertising for the US Senate and House races since January, Murdoch has given large sums to two of the three highest spenders.

The Republican Governors Association is the biggest spender, having lent support to Republican candidates with more than 19,000 adverts at a cost of $12m.

The US Chamber of Commerce, recipient of the latest News Corporation donation, had the third-largest spend, having invested $7m since January on more than 8,000 adverts backing Republican candidates.

Murdoch's decision to swing his vast financial resources – the latest Forbes rich list put his wealth at $6.2bn – behind the Republicans this year is more likely to be an indication of where he thinks the political wind is blowing than an ideological statement. His track record has been to follow probable election winners, rather than to make policy judgments.

In the UK, he switched from consistent Conservative support to back Tony Blair's Labour in the run-up to the 1997 general election.

In recent times in the US, he has expressed his admiration for Obama, who he called a "rock star", though his main news outlets, Fox News, the Wall Street Journal and the New York Post, all came out for Republican candidate John McCain in the 2008 presidential election.