WASHINGTON - In a key twist to the 2012 election, US President Barack Obama has reversed course and will encourage rich Democratic donors to back a fund that can raise unlimited millions for his re-election bid.

Obama's campaign manager Jim Messina said Obama would allow cabinet members and senior advisers to address big money events, reasoning that Democrats could not be "unilaterally disarmed" in the face of a Republican cash landslide.

Political committees known as super PACS have mushroomed since a 2010 Supreme Court ruling that gutted campaign finance laws and let corporations and individuals give unlimited sums to outside groups backing candidates.

A torrent of special interest money has poured into the Republican White House nominating race since the ruling, and Obama would have stood at a significant disadvantage if he did not conquer his qualms and back a super PAC.

The groups have unleashed a barrage of negative, damaging advertising and are certain to target Obama as he seeks a second term in November.

In one example of the new power of the super PACs, a group loyal to Republican front-runner Mitt Romney spent millions castigating his foe Newt Gingrich before January's Iowa caucuses.

Gingrich meanwhile has been kept in the race largely by a super PAC financed by a friend who is a casino tycoon.

"Our campaign has to face the reality of the law as it currently stands," said Messina in a message to Obama supporters, pointing out that Republican super PACs had spent $40 million in the last two months alone.

"With so much at stake, we can't allow for two sets of rules in this election, whereby the Republican nominee is the beneficiary of unlimited spending and Democrats unilaterally disarm."

Democratic sources had suggested that Obama believes the role of super PACs had been a corrosive influence on the already money-soaked world of US politics and that he had been reluctant to get involved.

But Messina said that the campaign would support the Priorities USA Action super PAC that was set up to support Obama but has so far failed to raise campaign cash in the huge quantities piled up by Republican candidates.

Though White House aides and officials will attend super PAC events, Obama, First Lady Michelle Obama and Vice-President Joe Biden will steer clear of such events and concentrate on raising cash for Obama's official campaign.

Agence France-Presse