US presidential contender Barack Obama urged Americans Sunday to overcome lingering barriers for black citizens, evoking the spirit of Martin Luther King at the church where the 1960s civil rights leader preached.

Fresh from a defeat by Hillary Clinton in a Democratic Party preference poll, Obama stopped by the Ebenezer Baptist Church in Atlanta to mark the late Reverend King's birthday and rally African-American voters behind a vision of national unity for political change.

"In the struggle for justice and for equality, we cannot walk alone," declared Obama, 46. "In the struggle to heal this nation and repair the world, we cannot walk alone. And so I ask you to walk with me."

Obama, who would be the nation's first African-American president, told the Sunday sermon audience that blacks also must address "deep-seated violence," anti-Semitism and anti-gay sentiment in their community.

"All of us will be called upon to make some sacrifice," he said.

Clinton, 60, beat Obama in Saturday's Democratic caucuses in Nevada, the latest in a series of state-by-state preference polls to determine the party's presidential candidate.

South Carolina, the first state with a major black electorate, holds the Democrats' next contest on January 26.

In a swipe at Clinton, Obama criticized other candidates who have mocked his core campaign message: that he can restore a sense of hope in the US when President George W Bush's eight-year term ends next January.

He won over the Atlanta congregation with his personal story - a multiracial child abandoned by his Kenyan father at age two, raised by his white mother from Kansas and rising to attend Harvard and win a US Senate seat.

"The odds of me standing here are so small, so remote, I couldn't have gotten here without some hope," he said.

He repeatedly drew distant parallels between his campaign and King's struggle for black civil rights and desegregated southern US states in the 1950s and '60s. King, a Nobel Peace laureate, was assassinated in Memphis, Tennessee, on April 4, 1968, sparking riots in dozens of US cities.

His January 15, 1929 birthday is being marked as an official holiday Monday. King's elder sister, Christine Farris King, was in the audience Sunday.

Obama centred on the nation's African-American minority, which traditionally leans heavily Democratic and may be crucial to his bid to win the party's nomination to run for president.

He cited a "moral deficit" in the US, including "profound structural and institutional barriers" that still exist for blacks and "the insidious role that race still sometimes plays on the job and in the schools, our health care, in our criminal justice system."

Overcoming the obstacles requires a national effort, including the money to pay for it, he said.

The church's pastor, the Reverend Raphael G Warnock, said Obama made plain that while some blacks died to win the right to vote, "now we can run for president."

"We invited this brother because he's committed, he's brilliant, he has a spiritual foundation and he is the embodiment of the American dream," Warnock said.