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

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

However, he added that he thinks this makes it “easier” to define her in the eyes of voters.

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

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

Trump is heading into debate day feeling — in his own words — “great.” The former president’s preparations for the debate were far less elaborate and structured than Harris’s, but he did participate in intense sessions over the past two days, a source familiar with the preparations said.

Three sources who speak to Trump regularly, including one directly involved in the debate preparation process, described Trump as nervous or disengaged from debate preparation in recent weeks. But that changed as the debate approached and he became much more informed.

The three sources, who spoke on condition of anonymity to describe private conversations with the candidate and his team, said he was much more focused on Tuesday than in recent days. Two sources suggested that recent positive public polls showing he could be a “change candidate” helped.

Two of the sources said Trump’s strategy will not be to paint Harris as a flip-flopper, but rather to take her most liberal previous stance and make it her own. 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 “is doing what she thinks is right to win the election.”

Trump has also changed his positions periodically over the years, including on issues like abortion, TikTok and even birtherism.

Harris, in turn, will try to “remind people what it was like under Donald Trump,” a source familiar with her debate strategy said.

Her team had tried in vain to keep the candidates’ microphones on the entire time. But even with the mics muted while the other person speaks, they’re bracing for unscripted and potentially explosive moments — including the possibility of Trump making disparaging remarks about her, according to sources familiar with her preparations.

Harris’ campaign did not immediately respond to a request for comment.