Washington (CNN) Former GOP Sen. Jeff Flake on Monday urged Republicans in Congress not to support President Donald Trump's 2020 reelection bid, saying they have a moral obligation to put their principles first -- even if it means risking their careers.

"My fellow Republicans, it is time to risk your careers in favor of your principles. Whether you believe the president deserves impeachment, you know he does not deserve reelection," wrote Flake, who represented Arizona before he left Congress earlier this year. "Our country will have more presidents. But principles, well, we get just one crack at those. For those who want to put America first, it is critically important at this moment in the life of our country that we all, here and now, do just that."

"Trust me when I say that you can go elsewhere for a job. But you cannot go elsewhere for a soul."

Flake, who decided to retire rather than seek reelection in 2018, had been a vocal critic of Trump during his last few years in office -- though he at times sided with the President, including last year when he provided critical support for Supreme Court Justice Brett Kavanaugh's confirmation.

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