Robert Mueller's testimony has started. Here's some background

Former special counsel Robert Mueller started his testimony before Congress on Wednesday morning. Democrats are hoping to glean something from this hearing that they can use against President Donald Trump, while some Republicans are hoping to undermine Mueller's credibility.

Here's what we know

Congressional leaders called Mueller in to get him to elaborate on the 448-page report he released in April on his yearslong investigation into whether or not Trump or his campaign colluded with Russia during the 2016 election.

Mueller had ultimately concluded that he could not find any evidence of collusion. However, a second section of the report detailed what Mueller said were attempts by Trump and people close to him to obstruct the investigation.

While Mueller has agreed to testify before Congress, he has insisted that he included all the relevant information about his investigation in his report. The Department of Justice has also instructed Mueller to limit the scope of what he tells Congress, with Associate Deputy Attorney General Bradley Weinsheimer warning him in a public letter to keep his testimony "within the boundaries of your public report."

Trump has been tweeting about the hearing

Trump released a flurry of at least eight tweets about the hearing Tuesday night and Wednesday morning. In his first of this series of tweets, Trump suggested that Mueller's choice of lawyer "should NOT be allowed" because this lawyer was a "Never Trumper."

He repeated this idea in another tweet, saying that this choice of lawyer "was specifically NOT agreed to, and I would NEVER have agreed to it. The Greatest Witch Hunt in U.S. history, by far!"

He also suggested that Mueller had lied about not wanting Trump to make him FBI director. Even if Mueller had wanted this job, he could never legally be FBI director again since he already served in that role for 12 years. FBI directors have been limited to 10-year terms since 1976, but Congress allowed Mueller to serve for two extra years. Mueller would have known this, so it is unclear why he would ask for a job he had no chance of getting.

It has been reported that Robert Mueller is saying that he did not apply and interview for the job of FBI Director (and get turned down) the day before he was wrongfully appointed Special Counsel. Hope he doesn't say that under oath in that we have numerous witnesses to the interview, including the Vice President of the United States!