Former Rep. Joe Crowley's own hubris led to his primary defeat at the hands of Rep. Alexandria Ocasio-Cortez — one of the biggest upsets in modern Democratic Party history.

Crowley, 57, was widely considered to be then-House Minority Leader Nancy Pelosi's successor within the caucus before his shock defeat last June to Ocasio-Cortez. But he held back from using damaging material against her because he thought he had a lock on New York's 14th Congressional District, according to a new book by Jake Sherman and Anna Palmer.

"Crowley had plenty of fodder he could've used against Ocasio-Cortez, but his top New York campaign operatives decided to take the punches and not hit back," Sherman and Palmer write. "It wasn't just that Crowley didn't want to go dirty; he thought it would be a sign of weakness in D.C. if he was seen in a tight race against Ocasio-Cortez. He was supposed to be the next Democratic leader, not someone who had to fight for reelection."

Ocasio-Cortez, 29, went on to beat Republican opponent Anthony Pappas, an economics and finance professor, last November in the general election, becoming the youngest woman ever elected to Congress.

Sherman and Palmer's book also chronicles Crowley's return to Washington two days after his loss and the reaction of his congressional colleagues, many of whom he had worked closely with over the course of his 20 years on Capitol Hill. The pair described the response as an "Irish wake," quoting Rep. Gerry Connolly, D-Va., who lamented how the House Democrats now had "no Plan B" to eventual speaker, Pelosi.

After disengaging from an embrace with Rep. Joe Kennedy of Massachusetts, Rep. Marcia Fudge of Ohio, and former Rep. Michelle Lujan Grisham, who now serves as New Mexico's governor, Crowley bumped into the family of Democratic 2020 presidential candidate, former Texas Rep. Beto O'Rourke.

"Crowley turned to see the children of Beto O'Rourke, the Texas Democrat who was looking to unseat Sen. Ted Cruz, and said, 'Your dad is going to be a great president someday,'" the duo write.

"The Hill To Die On: The Battle for Congress and the Future of Trump's America," published by Crown, is due for release on Apr. 9.