WASHINGTON — Republican Sen. Jeff Flake has a message for his party: Get a spine and stand up to President Donald Trump. It’s time for Congress to act as a brake on the president’s erratic impulses.

In a scolding book, Flake argues that his fellow Republicans are letting Trump lead the country away from conservative principles and staying silent as “the government of the United States has been made dysfunctional at the highest levels.”

As for Republicans controlling Congress, Flake warned in his new book, “Conscience of a Conservative,” that his party may be putting “at risk our institutions and our values” even as it faces the likelihood of scoring long-sought policy victories.

In the halls of the Senate on Tuesday, however, it didn’t seem as if Flake’s message was being heeded.

“My primary job is to work with the White House in the best interests of my constituents and the country, so I don’t really understand this argument that our job is to stand up to the president or to somehow oppose him,” said No. 2 Senate Republican John Cornyn of Texas.

“We’ve got challenges obviously when to try to get things done around here and we’ve got to be able to work with the administration in order to do that,” said Sen. John Thune, R-S.D., when asked to respond to Flake’s missive, most widely read as an except published by Politico Monday night. “So I think it’s fine that everybody has different opinions about where we are and where we need to go. He chose to express his and some members may choose not to express theirs.”

The Arizona Republican, narrowly elected to the Senate in 2012, has long been a critic of Trump, and it’s been rumored that Trump is itching to back a primary challenge to him next year. Unlike some Republican critics of Trump, whose voices became softer after his surprise victory, Flake has remained unsparing.

“To carry on in the spring of 2017 as if what was happening was anything approaching normalcy required a determined suspension of critical faculties. And tremendous powers of denial,” Flake wrote.

Flake sees himself as a conservative in the tradition of Arizona icon Barry Goldwater. The title of Flake’s book, “Conscience of a Conservative,” is the same title as Goldwater’s 1960 book.

Flake has built a strong political brand in Arizona, but his criticism of Trump and his fellow Republicans has already lit a fire back home.

“America — strong & unapologetic under @realDonaldTrump,” tweeted Flake’s GOP primary opponent Kelli Ward. “#globalists like Flake are the problem #FireFlake.”

Democrats praised Flake, a Mormon with a reputation as a straight shooter.

“I think he’s one of the finest people I’ve met in politics in terms of his principles and his morality,” said Senate Minority Leader Chuck Schumer, D-N.Y.

Other liberal commentators acidly observed that Flake has remained a loyal vote for GOP leaders and Trump, even as he worries that Trump’s penchant for bombast and ridiculing his opponents works against conservative goals.

Flake has voted for Trump’s nominees and efforts to reverse Obama-era rules, and he broke with fellow Arizona Sen. John McCain on last Friday’s health care vote. On one foreign policy issue, Cuba, Flake has been at odds with Trump, steadfastly supporting rapprochement with Havana.

“If we ascribe the worst motives to our opponents and demean them and call them clowns or losers, you just lose the ability to sit down and solve the big issues and actually enact conservative policy,” Flake told MSNBC’s “Morning Joe.” He added, “You know, somehow conservativism has become being mean or loud and you can’t enact conservative policy if you act that way.”

Flake said he’s particularly concerned conservative tenets of free trade and limited government have been lost to a national leadership that is erratic, mean-spirited and self-serving.

The senator says that since the election conservatives have been in denial as the government at its highest levels has become dysfunctional.

Flake says in his book that people who felt abandoned by the top parties were drawn to Trump, “a candidate who entertained them and offered oversimplified answers” to complex issues.

“If by 2017 the conservative bargain was to go along for the very bumpy ride because with congressional hegemony and the White House we had the numbers to achieve some long-held policy goals — even as we put at risk our institutions and our values — then it was a very real question whether any such policy victories wouldn’t be Pyrrhic ones,” Flake wrote.

Asked what would finally elicit vociferous objections from Republicans, Flake said the firing of special prosecutor Robert Mueller, who is investigating Russian interference in the 2016 presidential election.

“We can’t stand for that,” Flake said. “I don’t think Congress will.”

Flake’s book, published by Random House, went on sale Tuesday.