Former Arizona Sen. Jeff Flake, in an op-ed published in The Washington Post titled "Fellow Republicans, there's still time to save your souls," on Monday made the case for President Trump's impeachment.

To his former colleagues, he wrote: "Trust me when I say that you can go elsewhere for a job. But you cannot go elsewhere for a soul."

He then asked Senate Republicans to refuse to support a second term for Trump, claiming: "With what we now know, the president’s actions warrant impeachment."

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Democrats recently announced a formal impeachment inquiry into Trump over a whistleblower's claim he improperly pushed Ukrainian President Volodymyr Zelensky in a July phone call to investigate Joe Biden and his son Hunter, who served on the board of an energy company in Ukraine.

Hunter Biden served on the board of Ukrainian gas company Burisma as his father was leading the Obama administration’s diplomatic dealings with Kiev. Trump has claimed that Joe Biden pressured the Ukrainian government to shut down a corruption investigation of Burisma. The former vice president has denied doing anything wrong.

In the July call, Zelensky appeared to make an effort to stay in Trump’s good graces, telling him at least twice that he was “absolutely right.” But, after the White House released a rough transcript this week based on officials’ recollections of the call, Zelensky told reporters he didn’t feel pushed, emphasizing that “no one can pressure me.”

He also sought to play down the situation involving Biden and his son’s activities in Ukraine, calling it just one of “many cases that I talk about with leaders of other countries.”

"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.

"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."

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Flake retired from the Senate this year. He has frequently criticized Trump, as well as the Republican Party, since the president's election in 2016. Earlier this year, he called on others in his party to condemn the president’s recent controversial comments about several Democratic congresswomen.

He has also previously backed the idea of a Republican challenging Trump for the party's nomination in 2020.