Just minutes after a post apparently directed at Fox News, the president wrote: "Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG!
"A terrible thing. Read the Transcripts. This shoild never happen to another President again. Say a prayer!"
Mr Trump has now deleted the tweet and replaced it with a corrected version.
The president is expected to become the third president to be impeached in US history today over allegations that he withheld US military aid and a White House visit to Ukraine to force its leader into announcing an investigation into his 2020 election rival Joe Biden.
Although Mr Trump has denied any wrongdoing, multiple US diplomats and foreign service officials have testified about their concerns over his conduct.
The letter, which contained a string of grammatical errors, outlined why Mr Trump believes he should not be impeached and bizarrely compared his treatment to the Salem witch trials.
During the impeachment inquiry, Gordon Sondland, the US ambassador to the EU, directly implicated the president in his testimony and told Congress senior Trump administration officials were aware of inappropriate attempts to pressure Ukraine.
“Everyone was in the loop. It was not secret,” Mr Sondland said.
Representatives will vote on two formal articles of impeachment - abuse of power and obstruction of Congress.
Members of the House of Representatives are expected to vote mainly along party lines on Wednesday after bitter debates which have seen Republicans accuse Democrats of pursuing impeachment for partisan reasons.
If Mr Trump is impeached, he will face a trial in the Republican-controlled Senate, where senior figures have already indicated that they will not vote to remove the president.
Mitch McConnell, the Senate majority leader, has declared he is “not an impartial juror”, despite the fact that senators must take an oath swearing to do “impartial justice according to the Constitution and laws” at the start of an impeachment trial.
“Impeachment is a political decision,” Mr McConnell said.
“The House made a partisan political decision to impeach. I would anticipate we will have a largely partisan outcome in the Senate. I'm not impartial about this at all."
However, Republicans are expected to back a swift trial with few witnesses to shut down the impeachment process.
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