Republican Sen. Flake condemns Trump, says GOP should 'speak out' against 'erratic' administration The junior Arizona senator has been vocal in his criticism of Trump.

 -- GOP Sen. Jeff Flake, whose willingness to speak out against his party's presidential nominee last year caught the attention of then-candidate Donald Trump, is airing his grievances again, writing in a new book that his fellow Republicans are overlooking their responsibility to serve as a check on the executive branch.

Flake, the junior senator from Arizona, has a new book, "The Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle," and in an excerpt published Monday by Politico, he decries the dysfunction in "an erratic executive branch" and his colleagues' reticence to acknowledge Trump's shortcomings.

"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," writes Flake.

He notes the hesitancy within the GOP to fully embrace the businessman-turned-politician during the 2016 election cycle but also his fellow lawmakers' seeming lack of interest in serving as a check against Trump in the same way they did against a Democratic president.

"It was we conservatives who rightly and robustly asserted our constitutional prerogatives as a co-equal branch of government when a Democrat was in the White House but who, despite solemn vows to do the same in the event of a Trump presidency, have maintained an unnerving silence as instability has ensued," he writes.

Flake was critical of Trump during the presidential campaign and was one of the first sitting Republican lawmakers to publicly voice his disapproval of the nominee. In a notable encounter in July 2016, Trump clashed with Flake at a meeting with GOP senators, with Trump reportedly telling Flake he would lose his re-election bid, even though Flake was not on the ballot that year.

"I want to support our nominee. I really do," Flake said at the time. "But given some of the statements that have been made, I'm finding it difficult."

In the excerpt, Flake questions the meaning of the Republican Party's "political victories" if its members are willing to allow their principles to be "so malleable as to no longer be principles."

He urges the GOP "to speak out if the president 'plays to the base' in ways that damage the Republican Party's ability to grow and speak to a larger audience."