Former Republican Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (Ariz.) called on current GOP senators to put "country over party" when President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE's impeachment trial begins.

Flake, in a new op-ed for The Washington Post published Friday, told senators not to be "complicit" and warned that if they are, they "cede our constitutional responsibilities [and] set the most dangerous of precedents."

The warning comes just days after the House voted to charge Trump with abuse of power and obstruction of Congress, making him the third president in U.S. history to be impeached. The vote passed largely along party lines as Republicans have fiercely defended the president and his actions with Ukraine.

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Now all eyes are on the Senate when it returns from the holidays and starts an impeachment trial against the president. So far, top Senate Republicans have signaled their cooperation with the White House on the trial.

But Flake, a frequent Trump critic, advised his former colleagues to question if they'd conduct their trial the same way under former President Obama.

"My simple test for all of us: What if President Barack Obama Barack Hussein ObamaBiden says voters should choose who nominates Supreme Court justice Senate GOP set to vote on Trump's Supreme Court pick before election In a season of bad ideas, one stands out MORE had engaged in precisely the same behavior? I know the answer to that question with certainty, and so do you," Flake wrote. "You would have understood with striking clarity the threat it posed, and you would have known exactly what to do.

"But what is indefensible is echoing House Republicans who say that the president has not done anything wrong. He has," he continued. "If there ever was a time to put country over party, it is now. And by putting country over party, you might just save the Grand Old Party before it’s too late."

The scathing remarks come after Flake said earlier this year that Republicans should not support Trump's reelection.

Flake announced his retirement as a senator in October of 2017 after he prominently feuded with Trump. The president has taken credit for Flake's retirement.