It's the latest step into the political fray for King, who has a long history of colorfully weighing in on politics on both sides of the aisle going back half a century.
After serving nearly four years in prison for a manslaughter conviction in 1966 for killing an associate
who owed him $600, King was later pardoned
of the crime in 1983 by then-Ohio Gov. Jim Rhodes. The governor received letters of recommendation from several luminaries, including former presidential candidate Jesse Jackson, a Democrat, and then-Cleveland mayor (and future Ohio senator) George Voinovich, a Republican.
In 1992, King invoked his Fifth Amendment rights and refused to answer questions before a Senate subcommittee investigating ties between boxing and organized crime.
At the June funeral for Muhammad Ali, with whom King at one point worked, he declared his non-partisan status.
"I'm endorsing the people. I'm not a Republican or a Democrat, I'm a Republicrat, and I go with the will of the people," he told
the New York Daily News.
King previously praised Trump's Democratic presidential rival, Hillary Clinton, calling her "dynamic" at a 2014 fundraiser for Senate Minority Leader Harry Reid, a major Trump critic.
"I have been a supporter of Secretary Clinton for a long time," King said
at the event, which benefited the Nevada senator's re-election campaign.
"She is a dynamic woman, and I am a fighter for woman's rights," he added.
The Cleveland native supported President Barack Obama in both 2008 and 2012. At the time, he said white voters struggling to get behind a black candidate need to move past it.
King said they should "ask God to help you pretend that Barack Obama is white," he told
"I understand very well that after more than three centuries of being taught, conditioned and indoctrinated to hate the black man as your inferior, it is unrealistic to think that now you can just change to respect him," he wrote.
"Try to think of your beloved country America and what's best for Miss Liberty," King added. "Then try not to think of Barack Obama as a black man but as an American fighting for what's best for your children and your country."
King, The New York Times reported
, was supposed to address this summer's Republican National Convention, but several Republican National Committee officials, including chairman Reince Priebus, nixed the idea, citing the promoter's legal problems. King later said the party chairman removed him because he doesn't "like black people."
"I'm not speaking because Reince Priebus is still thinking he don't like black people," King told
reporters in July. "I have less than a damn about what Reince Priebus thinks, especially when he's so antiquated."
Black Lives Matter
King spoke on the tensions between black residents and law enforcement, saying the "root" of the problem gets lost in social media.
"You can't use short-term slogans because people don't understand and they twist them around," he said
on the floor of the Republican National Convention. "In our hashtag culture, we treat the effects, not the cause. You've got to get to the cause."
"You got to understand what I'm trying to say to you is that the white woman -- and I put it in this kind of (words) so you understand what I'm saying -- the white woman and the slave, the people of color," King said
Wednesday while campaigning for Trump. "When the system was created, they did not get heard. The first will be last and the last will be first."
"The white woman did not have the rights, and she still don't have the rights," he added. "And people of color don't have their rights -- those are the 'left outs.'"
"Donald Trump says no -- we (are) going back to inclusiveness," King added. "Everybody counts."
On which political party is best for black voters
"Martin Luther King says that both the Republican and the Democrat have a long history of using the black vote as a political football: Promise us everything, give us nothing," he told
New York Magazine in 2004.
George W. Bush
"Oooh! I get a passion. I'm getting excited right now when you said that. George Walker Bush -- he's tough-minded but he's tender-hearted. He's trying to reclaim that glory of that American Dream for all Americans. He put African-Americans in top policy positions higher than any president. He took two people, black, and put them in charge of 300 million people for their security," he told New York Magazine.
On Bush and terrorism
"George Bush utilizes the big stick. Whatever means necessary. If he wasn't in that White House when we had that despicable attack of terrorism in New York, I shudder to think, I tremble at what would be happening. We would probably have to be doing this interview on the underground, secretly, for fear if we'd go to the street, the treachery that abounds would take alight," he said in New York Magazine.