Forty years after Martin Luther King Jr. was shot to death in a racially charged assassination, the civil rights leader is still roiling American politics.

Democratic presidential candidate Hillary Clinton and Republican candidate John McCain have both come to Memphis to mark King's April 4, 1968, death and try to shore up support among black voters attracted to Democrat Barack Obama.

Both have some fence-mending to do among African Americans, and both are expected to give speeches and appear at an NBC News event to talk about King's leadership role in the 1960s movement against segregation.

Obama, who would be the first black U.S. president and is getting overwhelming support for his candidacy from black voters, will mark the holiday but will do it in North Dakota, where he will address the state's Democratic convention.

He was invited to the NBC event but could not attend due to a prior commitment, his campaign said.

Clinton, flying overnight from California and due to arrive before dawn, was accused of injecting race into the campaign when her husband, President Bill Clinton, was viewed as denigrating Obama in the South Carolina primary in January.

Clinton, who would be the first woman to win the White House, is scrambling to try to win the Democratic presidential nomination from Obama in what increasing looks like a difficult battle for her. Obama or Clinton will face McCain in the November election.

WORK TO DO

McCain has some work to do to improve his standing among black voters. He skipped a Republican campaign debate last September that focused on African-American issues.

And Democrats have been pointing out that Arizona Sen. McCain, as a member of the U.S. House of Representatives, voted in 1983 against creating a federal holiday marking King's birthday.

The holiday was approved by a 338-90 vote and President Ronald Reagan signed it into law.

McCain told reporters this week that he had "learned that this individual was a transcendent figure in American history" and deserved to be honoured.

He is to speak to the Southern Christian Leadership Conference at a civil rights museum built at the old Lorraine Motel where King was gunned down.

Obama, on the other hand, has been criticized by revelations about some of the sermons given by the Rev. Jeremiah Wright at Obama's Chicago church.

Wright, who has since retired, used inflammatory rhetoric from the pulpit, saying "God Damn America" as he railed against the country's history of racism.

Obama gave a well-received speech on race to try to allay concerns about why he sat in the pew all those years as Wright made outrageous statements.

Since those sermons came to light, some of Obama's supporters have suggested the Clinton campaign has been playing the race card against Obama.

This came up after Clinton said Wright would not be her pastor because of what he had said.

The McCain camp has declined to inject itself into the Wright controversy.