Former Republican Arizona Sen. Jeff Flake is once again speaking out against President Trump. In an opinion piece by Flake published in the Washington Post, Flake urged his former colleagues not to support the President's re-election bid. 

"My fellow Republicans, it is time to risk your careers in favor of your principles," Flake writes. "Whether you believe the president deserves impeachment, you know he does not deserve reelection."

The President and Flake have had a testy relationship in the past. Flake's critiques of the then candidate Trump continued even after President Trump took office. 

"I had not supported the president’s election. One year into his presidency, I knew that I could not support his reelection," Flake said.

Flake decided to retire from the Senate last year after serving for one six year term. Flake said he realized the political climate in his state did not bend in his favor. 

"I had hoped that I could still run for reelection to the Senate in 2018 as someone who would help to provide a check on the president’s worst impulses, it soon became apparent that this was not what Republican primary voters in my state were looking for," Flake writes.  

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Flake considered a primary challenge to Donald Trump for the Republican presidential nomination, but in January, Flake said he could not see a path to defeating him. 

The op-ed follows the opening of an impeachment inquiry in to the President, as well as the release of a transcript of a call in which President Trump repeatedly asks Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden, a potential 2020 presidential rival, and his son Hunter Biden.