Washington (CNN) President Donald Trump has repeatedly used his bully pulpit to attack, threaten and discredit witnesses who could testify against him.

On Wednesday, he turned his fire on John Bolton, Trump's former national security adviser who is now a key figure in the President's impeachment.

Bolton is poised to implicate Trump in the Ukraine scandal, confirming key allegations that he abused his power by withholding US aid from Ukraine as part of a pressure campaign to extract political favors. Senators are now weighing whether to issue a subpoena for Bolton's testimony.

With this new roadblock potentially stalling his anticipated acquittal, Trump lashed out at Bolton on Wednesday in a series of tweets. He accused his former national security adviser of lying, saying the allegations in his book are "nasty & untrue." He also accused Bolton of divulging "classified national security" secrets, even though Bolton's lawyer previously said the book doesn't contain any classified materials.

The impeachment drama is only the latest chapter in a series of investigations that have bogged down Trump's presidency. Since taking office, Trump hasn't been afraid to publicly meddle with these investigations, threatening witnesses and coming close to obstructing justice. Special counsel Robert Mueller investigated a wide array of Trump's actions as potential obstruction.

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