Trump's attacks against Bolton highlight past attempts to influence witnesses

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.

Here's a breakdown of Trump's past effort to influence and intimidate witnesses:

Trump tweeted out personal attacks against Marie Yovanovitch, the former US ambassador to Ukraine, while she testified on Capitol Hill. She blamed Trump for her untimely ouster from Kiev and said Trump's tweets were "very intimidating." During the impeachment trial, the Democratic House managers said this incident was part of Trump's "campaign of witness intimidation."

Throughout the impeachment process, Trump publicly threatened the whistleblower who filed the original complaint about Trump's dealings with Ukraine. Trump has goaded the press into revealing the whistleblower's identity, and said they were "close to a spy" who could be tried for treason. The whistleblower's lawyers said these comments put their client in danger.

Trump once ordered former White House Counsel Don McGahn to publicly lie about his efforts to fire Mueller. Federal prosecutors concluded that there was "substantial evidence" that this incident amounted to obstruction of justice, because McGahn was a key witness against Trump who later provided firsthand testimony about how Trump handled the Russia investigation.

Mueller concluded that Trump's comments about his former campaign chairman Paul Manafort "intended to encourage Manafort not to cooperate" with investigators, and also tried to influence jury deliberations. Trump publicly criticized the prosecutors and dangled the possibility of a pardon for Manafort, who is now serving a 7.5-year prison sentence for tax and bank fraud.

Trump used "attacks and intimidation" to deter his former lawyer Michael Cohen from testifying, which "could" have qualified as obstruction of justice, Mueller said. This included public threats against Cohen's family that, according to Mueller, were at least partially meant to "discourage" Cohen from implicating him. Cohen is serving a three-year prison term for tax fraud and campaign finance violations.

After Trump fired FBI Director James Comey, he tweeted a threat, saying Comey "better hope that there are no tapes of our conversations before he starts leaking to the press!" Comey later said this inspired him to go to the press with details of his conversations with Trump, in hopes of getting a special counsel appointed. Comey's firing was investigated as potential obstruction.

The special counsel investigated whether Trump obstructed the investigation into Roger Stone, a longtime friend who advised his 2016 campaign. The sections of the Mueller report about this conduct are redacted, but Trump publicly praised Stone when he declared he would never testify against the President. Stone was convicted of lying to Congress and witness tampering.