Global media mogul Rupert Murdoch has warned that a victory for Democratic White House hopeful Barack Obama next week may be "a real setback for globalisation."

Murdoch, the chairman of News Corporation, flayed Senator Obama's tax policies as "crazy", particularly his plan to hand out tax rebates to most Americans and to increase taxes for people earning more than $250,000.

He said Obama's promises to give tax rebates to 95 per cent of Americans was "rubbish" as "40 per cent of the US population don't pay taxes so how can he give them a tax cut."

However, the media tycoon was unsure if Obama would be able to implement all of the protectionist measures espoused by the party.

"Presidents don't often behave exactly as the campaign might have suggested because they become prisoners of all sort of things, mainly circumstances and events," Murdoch said in an interview with The Weekend Australian, owned by News Corp.

He warned that any rise in protectionism in the US, including introducing trade measures against China as espoused by some Democratic members in Congress, would risk retaliation and could threaten the world trading and financial systems.

"For the past three or four years, some Democrats have been threatening to do things like put on extra tariffs (against Chinese imports) if they don't change their currency," Murdoch said adding "if it happened, it could set off retaliatory action which would certainly damage the world economy seriously."