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After five days of controversy, Donald J. Trump called the former Ku Klux Klan sympathizer David Duke a “bad person,” as he condemned Mitt Romney’s planned speech about the real estate developer’s rise in the Republican Party.

Mr. Trump also pointed out that millions of new voters cast ballots in nominating contests since he declared his candidacy, and said if he would ever decide to bolt the Republicans to run a third-party race, they’ll come with him.

But Mr. Trump’s most significant comments were about Mr. Duke. On Sunday, Mr. Trump declined to disavow him despite being asked repeatedly by the CNN host Jake Tapper. Mr. Trump later insisted he had a bad earpiece from CNN as he gave the interview over a remote connection, although he appeared to have no problem hearing the questions. That interview came two days before Tuesday’s voting contests, in which Mr. Trump won seven states. Mr. Duke, a toxic figure for most Republicans, has praised Mr. Trump’s ascension in the Republican Party and has urged people to vote for him.

Finally, in an interview on MSNBC’s “Morning Joe” program on Thursday, Mr. Trump pushed Mr. Duke aside.

“David Duke is a bad person, who I disavowed on numerous occasions over the years,” Mr. Trump said. “And the one question that was asked of me, I guess on CNN, he’s having a great time, he talked about groups of people, and I don’t like to disavow groups if I don’t know who they are. I mean, you could have Federation of Jewish Philanthropies in groups. I don’t know who the groups are. He’s talking about disavowing groups. And that’s what I was referring to. But I disavowed him. I disavowed him. I disavowed the K.K.K.”

In fact, Mr. Trump had repeatedly said he couldn’t disavow specific white supremacist groups that Mr. Tapper asked about, saying he wouldn’t issue a blanket disavowal without knowing what the groups are. And his refusal to do so set off days of criticism from other Republicans.

Mr. Trump also gave a rebuttal of Mr. Romney’s planned speech in Utah on Thursday, saying that the 2012 Republican presidential nominee and former Massachusetts governor had begged him for an endorsement in his own race that year. And he warned the Republican Party, once again, that he could leave the race and run as a third-party candidate, as the window for such a run is closing.

“If I leave, if I go, regardless of independent, which I may do, I mean, may or may not, but if I go, I will tell you these millions of people that joined, they’ve told them, they’re all coming with me,” Mr. Trump said.