"The quality of his character is only more apparent," said Sen., a Republican who readily told a black audience that he had been wrong to vote against legislation making King's birthday a holiday.
"His good name will be honored for as long as the creed of America is honored," he said. "His message will be heard and understood for as long as the message of the gospels is heard and understood."
Like McCain, Democratic Sen.traveled to Memphis to observe the day. But unlike him, she chose not to speak at ceremonies at the Lorraine Motel where King was shot.
Instead, she was in the church where he had delivered his final sermon on the day before his death. A college student 40 years ago, Clinton recalled, "I walked into my dorm room and took my book bag and hurled it across the room." Her voice breaking, she added, "It felt like everything had been shattered and we'd never be able to put the pieces together again."
In a glancing reference to the current campaign, she added that "because of him, after 219 years and 43 presidents who have all been white men, this generation will grow up taking for granted that a woman or an African-American could be president of the United States."
Alone among the three, Sen.decided against a personal pilgrimage to the city of King's death. The strongest black candidate in history, he campaigned in Indiana, where he said King's pleas have yet to be answered fully.
"You know, Dr. King once said that the arc of the moral universe is long, but it bends toward justice. ... But here's the thing - it does not bend on its own. It bends because each of us in our own ways put our hand on that arc and we bend it in the direction of justice," said the Illinois senator.
"So on this day of all days let us each do our part to bend that arc. Let's bend it toward justice. Let's bend that arc toward opportunity. Let's bend that arc toward prosperity for all."
Race has been a constant, occasionally divisive companion to the Democratic campaign for the White House, and McCain's decision to speak at ceremonies held by the Southern Christian Leadership Conference was intended to demonstrate an eagerness to appeal to black voters who have long shunned Republicans.
For her part, Clinton has struggled to gain black votes in her competition with Obama. Additionally, some prominent black figures criticized her last winter for saying that it took a white man, President Lyndon B. Johnson, to finally win passage of the 1964 Civil Rights Act.
Obama's political imperative was different. He frequently recalls King's use of the phrase "the fierce urgency of now" as a motivation for his own candidacy for president. Yet particularly in the wake of controversy over his former pastor's rhetoric and his own major speech on race last month, he has sought to avoid narrowing his electoral appeal by being seen solely or even predominantly as a black candidate.
There was a scattering of boos as McCain spoke in front of the balcony where King was mortally wounded, but approving calls of "Amen" also floated up from a crowd huddled under umbrellas in a drenching rain.