In the book, an excerpt from which was obtained by The New York Times, Mr. Boehner writes that Mr. Trump’s “refusal to accept the result of the election not only cost Republicans the Senate but led to mob violence,” adding, “It was painful to watch.”

At another point, he writes, “I’ll admit I wasn’t prepared for what came after the election — Trump refusing to accept the results and stoking the flames of conspiracy that turned into violence in the seat of our democracy, the building over which I once presided.”

He adds: “Watching it was scary, and sad. It should have been a wake-up call for a return to Republican sanity.” Nodding to the divisions between the parties in Congress now, he writes, “Whatever they end up doing, or not doing, none of it will compare to one of the lowest points of American democracy that we lived through in January 2021.”

Mr. Trump, he goes on, “incited that bloody insurrection for nothing more than selfish reasons, perpetuated by the bullshit he’d been shoveling since he lost a fair election the previous November.” Mr. Boehner writes, “He claimed voter fraud without any evidence, and repeated those claims, taking advantage of the trust placed in him by his supporters and ultimately betraying that trust.”