"David Duke is a bad person, who I disavowed on numerous occasions over the years," Trump said on MSNBC's "Morning Joe."
"I disavowed him. I disavowed the KKK," Trump added. "Do you want me to do it again for the 12th time? I disavowed him in the past, I disavow him now."
The uproar started on Sunday when Trump was asked by CNN's Jake Tapper on "State of the Union" if he would disavow Duke and other white supremacist groups supporting his campaign.
"Just so you understand, I don't know anything about David Duke, OK?" Trump responded.
The next day, Trump blamed a "bad earpiece" during an appearance on NBC's "Today" show.
Trump also said Thursday he "may or may not" run as an independent if the Republican establishment continues to revolt against his possible nomination.
"If I leave, if I go, regardless of independent -- which I may do, may or may not -- but I will tell you these millions of people that joined [the GOP], they're all coming with me," Trump said.
Trump previously pledged to the Republican National Committee that he would not launch a third-party bid.
"So I signed a letter with the RNC and I said, 'I want to do this as a Republican' - the pledge, as they call it," Trump said. "But I'm not being treated the right way. I'm not being treated properly."
He added, "All of those people, whether I do independent or not...all of those people are going to leave the party. They're going to go back to where they came from."
Trump said he has brought out millions of new voters to the Republican Party and that he is the only GOP candidate who could beat the Democratic front-runner.
"I'm going to have more votes than anybody else. I brought millions and millions of people into the Republican Party in a short period of time. They only one that's going to beat Hillary Clinton is me," he said.
Despite spending millions on ads attacking him, Trump said the Republican establishment cannot successfully motivate his supporters to back another candidate no matter how hard they try.
"They are not joining [the GOP] for other people. They are joining for me. They aren't joining for Marco Rubio," he said.
The Republican front-runner could eventually "bury the hatchet" if the establishment began to recognize his contributions to the party.