U.S. President Donald Trump on Wednesday insisted he had done "nothing wrong" ahead of a vote in the House of Representatives, where he faces becoming the third U.S. leader ever to be impeached. 

"Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG! A terrible Thing," the president said on Twitter. 

"This should never happen to another President again. Say a PRAYER!" Trump wrote. 

Speaker of the Democratic-led House of Representatives Nancy Pelosi had announced on Tuesday that the body will vote on impeaching Trump over allegations he attempted to force Ukraine into investigating a main 2020 reelection rival, Joe Biden. 

He is also accused of obstructing Congress by refusing to cooperate with the impeachment investigation, barring staff from testifying and holding back documentary evidence. 

The two articles of impeachment are certain to pass in the House, where Democrats hold a firm majority. 

That will send the case to the Senate, where a trial of Trump is expected to open in January, and his acquittal is equally expected, given the Republicans' control there. 

On Tuesday, Trump sent Pelosi an angry six-page letter, in which he told the House speaker that "history will judge you harshly," while accusing her of "declaring open war on American Democracy." 

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While debate over impeachment rules goes on, Trump objects in letter