The battle over Rupert Murdoch's News Corporation is splitting American politics along party lines, with senior Democrats calling for investigations into the company while some senior Republicans play down the crisis.

Allegations that News International reporters may illegally have attempted to obtain the phone numbers of 9/11 victims have prompted an FBI probe into the company, which this week launched a preliminary investigation into the allegations. But a backlash has begun, focusing on the source of the claim: a single story in the Daily Mirror.

The former Republican presidential candidate Senator John McCain has criticised the decision to investigate, saying there is little evidence so far that the problem of phone hacking had spread to the US and the probe was unjustified. "It seems to me that this is a British issue that needs to be resolved first," he told Sky News. "I've heard of no evidence or allegation yet of anything being done in the United States of America."

His defence is significant not only because he is a senior Republican but also because he has criticised News Corp in the past. In 2001, he publicly questioned its plans to take over the satellite television service DirecTV, citing concerns over media consolidation.

On Saturday, the Washington Post criticised the Mirror report, saying it "appears to be based on a shaky foundation. The Mirror names no specific sources in its reporting, and it relies on a single anonymous second-hand source for its account."

More predictably, support has also come from News Corp's right-leaning cable channel Fox News, where there has been a reluctance to devote as much time to the story as other outlets, especially the left-leaning MSNBC network. A recent episode of the show Fox and Friends featured a media consultant, Robert Dilenschneider, who said that the scandal was being overplayed and Murdoch had "done all the right things".

Though one Republican congressman, Peter King, has been influential in calling for probes into News Corp, the bulk of the anger has come from Democrats. Four Democratic senators have written letters to the department of justice and the securities and exchange commission asking for investigations. They include powerful figures such as California senator Barbara Boxer and West Virginia senator Jay Rockefeller, who chairs the influential senate commerce committee.

At the same time John Podesta, a former Clinton chief of staff who is close to Obama, has come out swinging against News Corp. In an interview in Canada this week, he attacked Fox News and declared that the company might have broken US laws if it paid bribes to police in Britain. "This is not one rogue editor. This is an empire that was built on a set of journalistic ethics that's beginning to explode and unravel," he said.

Liberal campaigning groups have mobilised against News Corp: a protest was organised this week outside Rupert Murdoch's New York apartment, and there is a petition drive calling for an official investigation into the company. There has also been a focus on the company's political contributions, notably to the Republican Governors Association and the US Chamber of Commerce. That issue may prove less suitable for clear-cut campaigning, however, because News Corp has given money to politicians from both parties in the past.