John Boehner, the Republican former House speaker, issues a stinging denunciation in his new book of Donald Trump, saying that the former president “incited that bloody insurrection” by his supporters at the Capitol on Jan. 6 and that the Republican Party has been taken over by “whack jobs.”

The criticism from Boehner in his book, “On the House: A Washington Memoir,” represents an extraordinary public rebuke by a former speaker of the House toward a former president from his own party and shows how much the Republican Party has shifted since Boehner left Congress in 2015. And his remarks came as Trump has sought to retain his grip on Republican lawmakers’ loyalty from his new political base in South Florida.

Senate Minority Leader Mitch McConnell of Kentucky sharply criticized Trump at the end of the Senate trial for the former president’s second impeachment, pointing to his role in the Capitol riot. Others, like Rep. Liz Cheney of Wyoming, No. 3 in House Republican leadership, have also excoriated him.

But Boehner’s remarks went further, serving as a rejection of what the party he once helped lead. In the book, an excerpt from which was obtained by The New York Times, Boehner writes that 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.”

Trump, he goes on, “incited that bloody insurrection for nothing more than selfish reasons (ever) ... since he lost a fair election the previous November.” 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.”

In an emailed statement, Jason Miller, a spokesman for Trump, called Boehner a “Swamp Creature.”