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WASHINGTON — Arizona Sen. Jeff Flake lambasted President Donald Trump and other members of his party in a new book released Tuesday, offering the most in-depth, significant critique of Trump's presidency by a national Republican so far.

He was particularly critical of fellow conservatives who he says have failed to speak out against an "erratic" White House.

“That unnerving silence in the face of an erratic executive branch is an abdication, and those in positions of leadership bear particular responsibility,” Flake said in the book, "Conscience of a Conservative."

Just over six months into Trump's administration, Flake prognosticates that his party is engaged in the “spasms of a dying party.”

“We quickly set [inclusiveness] aside for the sugar high of populism, nativism, and demagoguery,” he writes. “The crash from this sugar high will be particularly unpleasant.”

Flake notes that any policy victories under Trump would be "pyrrhic ones." In an apparent willingness to take on the sitting president directly, Flake repeatedly critiques Trump's behavior and style of governance as unbecoming of a conservative, suggesting the "president's comportment was rather a study in the importance of conflict in reality television."

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"A conservative isn’t inconstant, mercurial, and shallow," Flake writes.

He continues: "In the absence of preparation and a well-considered strategy — especially when one is moving global chess pieces — volatile unpredictability is not a virtue."

Flake took his book's title from Barry Goldwater’s 1960 tome, also called "Conscience of a Conservative," which is oft-credited for laying the philosophical foundation for Goldwater’s GOP presidential nomination in 1964 and the eventual presidency of Ronald Reagan.

Flake is up for re-election next year and will face a primary challenge. But it’s not clear whether his book will prompt the president to personally wade into a potential effort to oust a sitting Republican senator.

Flake frequently criticized Trump's behavior on the campaign trail and publicly warned his party about the consequences of supporting Trump, who, in turn, called the senator "very weak and ineffective." In the book, Flake recounts meeting Trump last summer at a Senate GOP Caucus meeting on Capitol Hill, introducing himself as "the other senator from Arizona — the one who didn’t get captured."

Flake began writing the book in late November 2016 after a diplomatic trip to Mexico City. Notably quieter in criticizing the president since the election, Flake kept the book's development under wraps in the eight months since.

Now one year away from the primary for his re-election bid, Flake has just one Republican challenger in the race to date: Kelli Ward, a former state senator who unsuccessfully tried to unseat McCain last year.

The Trump political apparatus has engaged in conversations with Jeff DeWit, the former chief financial officer of the Trump campaign and current Arizona state treasurer, and Robert Graham, a financial adviser and former chair of the Arizona Republican Party, about potential runs.

“It’s definitely an option and consideration, but it’s whether all the pieces line up properly,” Graham told NBC News.

And DeWit, seen by Republicans in the state as the most formidable challenger, has given little indication that he intends to run. DeWit is one of the few notable Republicans in the state to have backed Trump’s candidacy.

Ward, the lone challenger, told NBC News that Trump voters in the state will not take well to Flake’s penchant for pushing back against the president, calling him one of the “original never Trump-ers.”

Josh Daniels, Flake’s campaign manager, said the senator has always “been honest when he disagrees with any president.”

“But he is proud to support policies that will make Arizona stronger, and that’s why he’s voted in favor of legislation supported by President Trump over 95 percent of the time this year,” Daniels added.

Flake could also face a competitive election in November. Congresswoman Kyrsten Sinema and Phoenix Mayor Greg Stanton are considering runs for office. Flake beat his Democratic challenger in 2012 by four percentage points.