Donald Trump caught former Vice President Kamala Harris and her aides off guard when the typically abrasive president politely praised her as a “tough cookie” and congratulated her on running a “great” campaign when she called him to concede his victory in the 2024 election, according to a new book.

“I’m calling to concede. It was a fair election. The peaceful transfer of power is important. It’s important to the country,” Harris reportedly told Trump on the phone the day after the election, according to author Chris Whipple’s new book “Uncharted: How Trump Beat Biden, Harris, and the Odds in the Wildest Campaign in History.”

However, she could not help but take a light jab at the divisive president-elect at the time, adding “I hope you’re president for all Americans.”

Instead of taking that comment personally and snapping back, however, Trump was cordial and even praised Harris’ late-entry campaign as he basked in his historic win, according to Whipple.

“You’re a tough cookie. You were really great. And that Doug — what a character! I love that guy,” the now-president said, referring to Harris’ husband, Doug Emhoff.

One of Harris’ aides, however, found the president’s tone — a complete change from his campaign rhetoric — “odd and inappropriate.”

“I was like, what?” the unidentified aide barked. “Honestly, I felt like, what is this? It’s so manipulative. He’s a sociopath.”

The traditional concession call almost never happened, however, due to technical difficulties — adding to the awkwardness of the moment, Whipple said in the book, according to an excerpt obtained by the Daily Mail.

Harris’ campaign Chief of Staff Sheila Nix and her White House Chief of Staff Lorraine Voles set up the call from Harris’ Naval Observatory home in Washington, DC — but when she phoned Trump, the calls wouldn’t merge with Harris’ phone.

Instead, the Voles called Harris from her phone, and she and Nix “pressed the phones together,” Whipple wrote.

As Harris heard Trump’s voice come through, she “felt the weight of the crushing defeat,” according to Whipple.

Whipple’s new book will hit the shelves Tuesday.