PHILADELPHIA – Former President Donald Trump said Tuesday his biggest challenge in debating Vice President Kamala Harris was that she changed her positions on so many issues.

“You don’t know what to expect. She’s changed all her policies over the years,” Trump said in a telephone interview with NBC News on Tuesday morning, a few hours before the debate here.

However, he added that he believed this would make it “easier” to define them in the minds of voters.

“It makes it a lot easier. She’s no longer credible,” Trump said.

The debate, sponsored by ABC, is Trump’s second in the 2024 election – but his first against his new opponent. For Harris, it is the first general election debate as a presidential candidate.

Trump is entering debate day feeling, as he himself puts it, “great.” The former president’s preparations for the debate were far less extensive and structured than Harris’s, but he attended intensive meetings over the past two days, according to a source familiar with the preparations.

Three sources who regularly speak to Trump, including one who was directly involved in debate preparation, described Trump as nervous or disinterested in debate preparation in recent weeks. But that changed as the debate approached, and he was much more focused.

The three sources – who asked not to be identified to describe private conversations with the candidate and his team – said he was in a much more focused state on Tuesday than in recent days. Two sources said recent positive polls showing he could be a candidate of “change” helped.

Two of the sources said Trump’s strategy will not be to portray Harris as fickle, but rather to take her most liberal position from the past and make it her own. One person pointed to comments from Sen. Bernie Sanders (I-Vt.) as a roadmap. On Sunday, he said on NBC News’ “Meet the Press” that Harris is a progressive but “does what she thinks is right to win the election.”

Trump has also changed his positions frequently over the years, including on issues such as abortion, TikTok, and even the birther movement.

Harris, for her part, will try to “remind people what it was like during the Donald Trump years,” according to a source familiar with her debate strategy.

Her team had pushed in vain to keep the candidates’ microphones on the entire time. But even if the microphones are muted while the other person is speaking, they are preparing for unforeseen and potentially explosive moments – including the possibility of Trump making disparaging remarks about them, according to sources familiar with their preparations.

The Harris team did not immediately respond to a request for comment.