Barack Obama was to accept his historic nomination on Thursday as the first black candidate to top a major US political party's presidential ticket, delivering on the closing night of the Democratic convention an impassioned call for change in a country where exactly 45 years earlier civil rights leader Martin Luther King challenged Americans to embrace his "dream" of equality.

Obama, who has made little of his race in a so-far bruising run for the White House, was sure to include his personal story in his acceptance address before 75,000 fellow Democrats at a Denver stadium, and millions more watching on television.

But the nominee - the son of a black Kenyan father and a white American mother - was also to talk about the US's many challenges, from health care to international threats, campaign manager David Plouffe said on ABC's Good Morning America news show.

His acceptance of the Democratic nomination comes on the 45th anniversary of the Rev King's August 28, 1963, "I Have a Dream" speech, an exhortation about the frustration of blacks at a time when African Americans in many southern states were denied their voting rights more than 90 years after the federal government guaranteed them that right.

Given America's tortured racial history - Obama was just 2 when King delivered his speech - the candidate's nomination is a gamble for the Democrats in the November 4 election as they work to wrest the White House from the Republicans and their candidate McCain, a veteran Arizona senator and Vietnam war hero who turns 72 on Friday.