Former President Donald J. Trump was “really depressed” in the days after losing re-election and leaving office in January 2021, so much so that he was “not eating.”

At least that is what Kevin McCarthy told Liz Cheney in trying to explain why he had traveled to Mr. Trump’s Mar-a-Lago estate in Florida, an act of solidarity that many have identified as a pivotal moment in reviving the former president’s political viability.

Mr. McCarthy, the California congressman who was then the House Republican leader, had condemned Mr. Trump for fueling the Jan. 6 mob attack on the Capitol and even suggested that he resign, only to turn around and effectively absolve the former president by embracing him again. In her new book, Ms. Cheney, perhaps the country’s most vocal anti-Trump Republican, reports that Mr. McCarthy justified the Jan. 28 visit as an act of compassion for a beaten ally.

Ms. Cheney wrote that she was so shocked when she first saw the photograph of Mr. McCarthy and Mr. Trump standing side by side with grins on their faces that she thought it was a fake. “Not even Kevin McCarthy could be this craven, I thought,” she wrote. “I was wrong.” She went to see Mr. McCarthy to confront him about rehabilitating the twice-impeached former president who had just tried to overturn an election he lost.