The President has escalated his attacks on the Mueller probe in recent days, and his tweet on Wednesday comes on the second day of former Trump campaign chairman Paul Manafort's trial, the first case Mueller's team has taken before a jury.
He added: "There are two things specifically that are worrying Donald Trump at the moment".
"He's loathed Jeff Sessions ever since he recused himself from the whole issue of Russian Federation meddling".
Manafort, the lone American charged by Mueller who has opted to stand trial instead of cooperate with prosecutors, was described by his defence lawyer as a hugely successful global political consultant who left the details of his finances to others.
"Jeez", gasped Republican Senator Susan Collins of ME as she was read the president's tweet in an interview. If Mueller can not make out an obstruction claim now, I'd be very surprised.
After Democrats claimed the tweet amounted to obstruction of justice, Sanders said the president was not actually giving an order to end the probe. In a series of tweets, Trump called on Jeff Sessions to put a stop to the investigation.
Some legal analysts said Wednesday's tweet alone was not enough to bring an obstruction of justice charge but it could be used to establish a pattern of conduct revealing Trump's intentions.More news: USA proposes higher tax on $200 billion in Chinese imports
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Prosecutors have portrayed Mr Manafort as a tax cheat who hid money in offshore accounts, and lied to borrow millions more against real estate in a bid to maintain an extravagant lifestyle once the work dried up.
Senator Susan Collins said it "was entirely inappropriate and intemperate" for Trump to make such a request of Sessions.
The special counsel is also scrutinising whether there were any attempts by the president to obstruct justice.
It's apparent that Donald Trump is feeling the encroaching Muller probe closing in on his campaign. House Intelligence Committee ranking member Rep. Adam Schiff said the tweet amounted to "an attempt to obstruct justice hiding in plain sight".
On Monday, I recapped Trump's insane tweets over the weekend, as they painted a picture of a man truly rattled by Robert Mueller's investigation.
After the judge asked Asonye for a clarification, Asonye said prosecutors are constantly evaluating the need to call a particular witness and his comments were "not to suggest we are not calling him".
Gingrich said Trump should "endure" the investigation instead of firing Sessions, but that Sessions has the "absolute authority" to fire Mueller's team. Several Republican lawmakers said Mueller should be allowed to complete his investigation. On Wednesday, the president distanced himself from his former campaign chief, saying that he worked for him "for a very short time".
Manafort was the campaign chairman for about five months in 2016.