By Caren Bohan

DENVER (Reuters) - Barack Obama burst onto the national stage four years ago with a speech describing himself as a skinny guy with a funny name and an improbable life story.

On Thursday night, in a return to the Democratic convention stage, the 47-year-old U.S. senator takes his place in the history books as the first black presidential nominee of a major political party.

For many he embodies the American Dream in which anybody can grow up to be president, including the son of a Kansas mother and an absent Kenyan father with an exotic name who spent his early years living abroad.

"In no other country on earth is my story even possible," Obama said in his keynote speech at the last Democratic party convention in 2004.

His oratorical gifts were on full display in that speech and have become Obama's trademark, launching him to instant fame and setting the stage for his White House bid in the November 4 election against Republican John McCain.

On Thursday, the first-term senator from Illinois will accept the Democratic presidential nomination before a crowd of 75,000 people at the giant Invesco football stadium in Denver.

The event coincides with the 45th anniversary of civil rights leader Martin Luther King's "I Have a Dream" speech. That 1963 speech is seen as an expression of the country's highest aspirations of equality and justice.

Many Democrats see Obama's campaign as a step towards bridging the country's racial divide -- black Americans constitute some 12 percent of the population -- but he has made a point of not running on his race.