The American actor and activist, who travelled by car from his residence on Lake Como, Italy, arrived in Geneva's Old Town ahead of a private reception followed by a gala dinner.

About $500,000 could be raised for Obama's campaign coffers as he battles to keep pace with the fundraising of his rival, Republican Mitt Romney, sources close to Obama's campaign said.

At least 100 American supporters paid $1,000 each to attend the champagne reception being held in a museum, while about 35 high-rolling donors paid $15,000 a plate for the gourmet dinner with Clooney, the sources said.

Democrats Abroad Switzerland, the U.S. party's Swiss branch, said in a statement the event was expected to be the "single most important fundraising event" outside the United States for Obama's campaign.

Some participants said it would be the Democrats' biggest fundraising event abroad to date.

Clooney, 51, was to address the gathering, along with Matthew Barzun, chairman of the Obama Victory Fund, and organizer Charles Adams, a Geneva-based American lawyer on the National Finance Committee of the campaign who declined comment.

Supporters willing to pay $5,000 get to have their photograph taken with the Hollywood star and director - who was not accompanied by his latest girlfriend Stacy Keibler.

"OUTSPENT AND OUTRAISED"

Clooney hosted a glitzy campaign fundraiser at his home in Los Angeles in May, raising nearly $15 million at the event where Obama called his endorsement of same-sex marriage a "logical extension" of his vision for the country.

The Geneva fundraiser is a virtual carbon copy of one attended here by Clooney four years ago that raked in hundreds of thousands of dollars to help take Obama to the White House.

Obama's campaign to win re-election in the November 6 poll has struggled to match the enthusiasm of his 2008 run and compete financially with the hundreds of millions of dollars being raised by conservative outside groups known as super-PACs.

In July, Romney and his Republican Party raised $101 million, outpacing Obama for the third month in a row. Obama and the Democrats hauled in $75 million.

Romney and his party said they had $186 million left in cash on hand at the end of July - almost $60 million more than Obama and his party, who had $127 million left in cash on hand, disclosures filed last week showed.

"We're being outspent and outraised. It's everyone to their battle stations, we've got to keep the playing field as level as possible," a participant at the Geneva fundraiser told Reuters. "Clooney has made himself available to an extent above and beyond the call of duty."

Tropical Storm Isaac forced Republicans on Sunday to postpone the start of the Republican National Convention being held this week in Tampa, Florida, where the party will officially nominate Romney as their candidate.

(Reporting by Stephanie Nebehay; Editing by Pravin Char)

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