(CNN) Hillary Clinton casts Bernie Sanders as an unrealistic over-promiser in her new book, according to excerpts posted by a group of Clinton supporters.

She said that his attacks against her during the primary caused "lasting damage" and paved the way for "(Donald) Trump's 'Crooked Hillary' campaign."

Clinton, in a book that will be released September 12 entitled "What Happened," said Sanders "had to resort to innuendo and impugning my character" because the two Democrats "agreed on so much."

The excerpts represent a small number of the roughly 500-page book in which Clinton reflects on her stunning loss to Donald Trump in 2016. The few pages on Sanders are also remarkably candid for a candidate who was more known for being careful than blunt, especially when it came to the Vermont senator.

A Clinton spokesman declined to comment on the published excerpts.

"Some of his supporters, the so-called Bernie Bros, took to harassing my supporters online. It got ugly and more than a little sexist," she wrote.

"When I finally challenged Bernie during a debate to name a single time I changed a position or a vote because of a financial contribution, he couldn't come up with anything," Clinton wrote. "Nonetheless, his attacks caused lasting damage, making it harder to unify progressives in the general election and paving the way for Trump's 'Crooked Hillary' campaign."

The excerpts were first posted by a pro-Clinton Twitter account and by a user on a Clinton "super volunteer" Facebook page.

"She says a lot in this book, and some of it is going to surprise people. People should buy it, read it, and consider what she constructively lays out. It's a great read," a Clinton aide said, asking not be named because they were not authorized to discuss the book.

Clinton's decision to step back into the spotlight with the book will likely be met with wide praise from many in the Democratic Party, including some of the millions of Democrats who backed her over Trump. But it also could tear at wounds that are still open between the wing of the party Sanders animated and those who backed Clinton.

While Clinton remains a powerful force among Democrats -- and will be in the coming elections -- there are some who have publicly said they would rather the party move on from the 2016 election.

Additionally, Clinton's public critique of Sanders has her attacking a politician who has stoked speculation that he could run for president against in 2020.

"I am not taking it off the table," Sanders said in July.

In her forthcoming book, Clinton noted that the Vermont independent "isn't a Democrat."

"That's not a smear, that's what he says," she wrote. "He didn't get into the race to make sure a Democrat won the White House, he got in to disrupt the Democratic Party."

After outlining how she disagrees with Sanders' view of the Democratic Party, Clinton concludes, "I am proud to be a Democrat and I wish Bernie were, too."

Clinton wrote that President Barack Obama counseled her to "grit my teeth and lay off Bernie as much as I could," according to the excerpts. That strategy, Clinton wrote, made her feel she was "in a straitjacket."

She noted that Jake Sullivan, her top policy aide, told her that Sanders' campaign strategy reminded him of a scene from the movie "There's Something About Mary," where a hitchhiker says he has a plan to roll out seven-minute abs to top the famous eight-minute abs.

"Why, why not six-minutes abs?" Ben Stiller's character asks.

Clinton wrote: "That's what it was like in policy debates with Bernie. We would promise a bold infrastructure investment plan or an ambitious new apprenticeship program for young people, and then Bernie would announce basically the same thing, but bigger. On issue after issue, it was like he kept promising four-minute abs, or even no-minutes abs. Magic abs!"