If elected, he said, he would work to resolve such issues as an unpopular war in Iraq and health and public education systems he said leave too many people behind.
"We can do all these things. I can't do it without you," Obama told the audience of several thousand as supporters, many waving blue "Obama '08" signs.
Among the stars donating to the Obama campaign are George Clooney, Eddie Murphy, Barbra Streisand, Denzel Washington and Tom Hanks.
Big names
Obama is seen as the best hope yet for a black
candidate in the White House [Reuters]
Such donations do not automatically bring with it mass voter appeal, but the cash is vital with the cost of a presidential campaign running well into the tens of millions of dollars.
Previous campaigns have often seen big names donating to multiple campaigns while withholding formal endorsements until later.
Obama, a senator from Illinois, formally launched his campaign for the presidency on February 10, calling for an end to the war in Iraq and the withdrawal of all US troops by March 2008.
His campaign has drawn huge media attention in the US and around the world.
But it is still several months until the first primary - and well over a year until the presidential election in November 2008 - and the crucial question remains whether his campaign can sustain the early momentum it has generated.