'You don't know what to expect': Trump outlines his biggest challenge in debating Harris

PHILADELPHIA — Former President Donald Trump said Tuesday that his biggest challenge in debating Vice President Kamala Harris is that she has shifted her positions on so many issues.

"You don't know what to expect. She's changed all of her policies over the years," Trump told NBC News in a phone interview Tuesday morning hours before the debate here.

He added, however, that he believes that makes it "easier" to define her in the minds of voters.

"It makes it much easier. She's no longer believable," Trump said.

The debate, sponsored by ABC, is Trump's second of the 2024 election — but his first against his new opponent. It's Harris' first general election debate as a presidential candidate.

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Trump enters debate day feeling — in his words — "great." The former president's debate preparations have been far less extensive and structured than Harris', but he participated in intensive sessions over the past two days, according to a source familiar with the preparations.

Three sources who speak to Trump regularly, including one directly involved in the debate prep process, described Trump as either nervous or disengaged with debate preparation in the past few weeks. But that changed as the debate grew closer, and he's been much more dialed in.

The three sources — who requested anonymity to describe private conversations with the candidate and his team — said that he was in a much more focused head space Tuesday than he has been in recent days — with two suggesting that recent favorable public polling showing that he could be a "change" candidate has helped.

Two of the sources said that the Trump strategy will not be to paint Harris as a flip-flopper, but rather to take her most liberal past position and make her own it. One person pointed to comments by Sen. Bernie Sanders, I-Vt., as a road map. On Sunday, he said on NBC News' "Meet the Press" that Harris is a progressive but “doing what she thinks is right in order to win the election."

Trump, too, has frequently shifted his positions over the years, including on issues like abortion, TikTok and even birtherism.

Harris, for her part, will try to "remind people of what it was like during Donald Trump’s years," according to a source familiar with her debate strategy.

Her team had unsuccessfully pushed to have the candidates' microphones on the whole time. But even with the microphones muted while the other person is speaking, they're preparing for unscripted and potentially volatile moments — including the possibility that Trump makes derogatory comments about her, according to sources familiar with her preparations.

The Harris campaign did not immediately return a request for comment.

This article was originally published on NBCNews.com