LOS ANGELES: George Clooney, who hosts a star-studded fundraiser for Barack Obama Thursday, is a Hollywood A-lister turned activist for liberal causes like Sudan, Haiti - and now the US leader's re-election.

The movie heartthrob's $40,000-a-plate dinner is being tipped as possibly the biggest presidential fundraiser ever, bolstering Obama's coffers to the tune of $12 million as he gears up for a fierce re-election fight.

Clooney will welcome at least 150 people at his 7,400-square foot home in Studio City, where catering and security preparations have been underway for days. Locals are bracing for snarled traffic when the celebrities descend on Clooney's mansion.

The 51-year-old Oscar-winning actor has long been used to turning heads, but his stature has been further enhanced by his campaigning activities of recent years, arguably making him Obama's celebrity activist-in-chief.

It is all a long way from the young actor who made his name as a doctor in TV medical drama “ER” in the 1990s.

Even though his first big-screen lead role, in 1997's “Batman and Robin,”was widely panned, Clooney became an A-list star with role in the high-seas blockbuster “The Perfect Storm” in 2000.

In that year he formed his own production company with director Steven Soderbergh, Section Eight Productions, and together they turned out such hits as 2001's “Ocean's Eleven,” followed by sequels in 2004 and 2007.

But Clooney also broached more thoughtful fare like in the 2005 spy thriller “Syriana,” which earned him a best supporting actor Oscar, and the black-and-white film “Good Night, and Good Luck” about freedom of speech and governmental abuse of power.

His growing political interests also came to the fore in last year's primary season thriller “Ides of March,” while Hawaiian-set family drama “The Descendants” won a best adapted screenplay Oscar.

Clooney's rise has been matched by an increasing activism.

He has campaigned tirelessly to draw attention to the conflict in Sudan's Darfur region. In 2010 he was named a United Nations Messenger of Peace by Secretary General Ban Ki-moon in recognition of his work.

Also that year Clooney was one of the driving forces behind a star-studded telethon which raised more than $58 million for victims of the Haiti earthquake.

In March this year Clooney was handcuffed and arrested along with several members of the US Congress outside Sudan's embassy in Washington DC, as they demanded an end to an offensive they fear will cause thousands to starve.

He was released three hours later after paying a $100 fine for crossing a police line.

In the same month Clooney visited the White House to brief the president on a clandestine visit he had made to war-torn Sudan. He also attended a state dinner for British Prime Minister David Cameron.

Obama appears to be keen on wooing wealthy donors in Hollywood and Silicon Valley, as money from Wall Street may be harder to come this election cycle as he gears up to take on likely Republican nominee Mitt Romney in November.

Tim Groeling, an associate professor at the University of California at Los Angeles (UCLA), said Clooney's fundraiser was important both for the cash raised and the impact of his celebrity endorsement.

“Most successful Hollywood entertainers are well-liked by the public, or else they wouldn't have been successful in the first place,” said Groeling, UCLA's head of communication studies and an expert on political communications.

“Although they are not always regarded as politically knowledgable, if someone you like endorses something or someone, that connection should be at least somewhat credible to you,” he told AFP.

Organization for Thursday's dinner has been spearheaded by DreamWorks Animation chief Jeffrey Katzenberg, the president's major entertainment industry fundraiser, according to the Hollywood Reporter.

The soiree is expected to make $5-6 million, while a further $6 million had been made by an online sweepstakes, in which the winner gets two seats at the head table with Obama and Clooney, the industry paper reported.