SCOTTSDALE, Ariz. — Before Bob Corker, there was Jeff Flake.
Mr. Flake, the even-tempered Republican senator from Arizona, has for months offered stinging critiques of President Trump’s character, demeanor and truthfulness — the same message forcefully echoed a week ago by Mr. Corker, a Republican colleague from Tennessee, who warned that Mr. Trump’s reckless behavior could lead to “World War III.”
But there is one crucial difference between the two: Mr. Flake, unlike Mr. Corker, is running for re-election. And now he finds himself in grave political peril.
Mr. Flake is perhaps the most endangered Senate Republican, with an approval rating in one recent poll of just 18 percent among Arizonans. Mr. Trump has savaged Mr. Flake as “toxic” and a “flake,” and has encouraged a primary challenge against him that has left the senator squeezed not only from the left but also the right.
His fate is an object lesson for other Republicans who might consider voicing dire thoughts about the president’s fitness: Cross Mr. Trump, and your political career could well be over.
Mr. Flake, who is known more for his decency than his independent streak, said he had no regrets.
In an interview here, he ticked off some of his earliest criticisms of the president — from the days when Mr. Trump peddled the false theory that President Barack Obama was born in Kenya, to the time Mr. Trump referred to Mexican immigrants as “rapists,” to his call for a complete ban on travel to the United States by Muslims — before looking up and stopping himself.
“In which of those instances,” the senator asked, “should I not have spoken out? At what point should you not stand up and say, ‘This is not right; this is not conservative; this is not where Republicans ought to be?’”
Mr. Flake said he had known from the start that taking on Mr. Trump might do him political harm. Even before he declared the president’s brand of populism a corruption of conservative values, he anticipated a tough primary challenge, given his policy differences with Mr. Trump on issues like immigration, trade and Cuba.
“The truth is, if my only goal were to be elected, re-elected to mark time in the Senate, there are much easier paths,” he said.
Mr. Flake is not the Senate’s only vulnerable Republican; Senator Dean Heller of Nevada is also facing a tough re-election race. And Republicans will now have to field a candidate to succeed Mr. Corker, who announced late last month that he was not running next year.
Last weekend, Mr. Corker said his concerns about Mr. Trump were shared by nearly every Senate Republican, even if few have spoken out. Mr. Flake, by contrast, has put pen to paper with his criticism; his new book, “Conscience of a Conservative,” published in August, is a blistering indictment of the Republican Party and of a president who, despite record-low overall approval ratings, has retained the support of about 80 percent of his party.
Mr. Flake’s main primary challenger at the moment, Kelli Ward, made clear in an interview that she intended to paint Mr. Flake as “an obstructionist to the America First agenda that Donald Trump touted on the campaign trail, and that the American people want to see enacted.”
Ms. Ward, an osteopathic physician and a former state senator who ran unsuccessfully against Arizona’s other senator, John McCain, in 2016, was busy preparing last week for her campaign kickoff. It is scheduled for Tuesday night with the conservative radio host Laura Ingraham as the featured guest.
Andy Surabian, senior adviser to the Great America Alliance, a Trump-aligned group whose political action committee has been supportive of Ms. Ward, said Mr. Flake’s troubles were “entirely self-inflicted.”
“If Flake wants to know why he’s vulnerable, all he needs to do is look in the mirror,” said Mr. Surabian, who had a stint in the White House as deputy to Stephen K. Bannon, Mr. Trump’s former…