President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE said Sunday that former Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE "should be ashamed of himself" for allowing the special counsel's investigation into Russian interference to proceed, levying yet another attack on one of his favorite targets for criticism.

Trump issued a series of tweets attacking the investigation as a "Democrat scam" that is "very bad for our country," and pushing the unproven theory among some conservatives that Mueller's team has led witnesses to make false statements.

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"People are starting to see and understand what this Witch Hunt is all about," Trump added. "Jeff Sessions should be ashamed of himself for allowing this total HOAX to get started in the first place!"

“It looks here as though General Flynn’s defenses are incidental to something larger which is for the prosecution to figure out if it can find a path to Donald Trump without quite knowing what that crime might be. It stops looking like prosecution and more looking like...... — Donald J. Trump (@realDonaldTrump) December 16, 2018

....a persecution of the President.” Daniel Henninger, The Wall Street Journal. Thank you, people are starting to see and understand what this Witch Hunt is all about. Jeff Sessions should be ashamed of himself for allowing this total HOAX to get started in the first place! — Donald J. Trump (@realDonaldTrump) December 16, 2018

....The Russian Witch Hunt Hoax, started as the “insurance policy” long before I even got elected, is very bad for our Country. They are Entrapping people for misstatements, lies or unrelated things that took place many years ago. Nothing to do with Collusion. A Democrat Scam! — Donald J. Trump (@realDonaldTrump) December 16, 2018

Mueller's investigation has thus far implicated former Trump associates Michael Flynn, George PapadopoulosGeorge Demetrios PapadopoulosTale of two FBI cases: Clinton got warned, Trump got investigated Trump says he would consider pardons for those implicated in Mueller investigation New FBI document confirms the Trump campaign was investigated without justification MORE, Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE, Richard Gates and Michael Cohen, as well as more than 20 Russians.

Cohen, who for years worked at the Trump Organization, was sentenced last week to three years in prison for bank fraud, tax fraud and campaign finance violations he said he committed at Trump's direction.

The president has denied directing Cohen to break the law.

Flynn, who pleaded guilty to lying to the FBI last year as part of Mueller's investigation, has been a figure of conservative sympathy in recent months.

Trump and others have seized on a suggestion from Flynn's defense attorneys that Trump's former national security adviser had been duped by FBI agents who handled his interview to argue Flynn had been wrongly led to commit a federal crime.

In a court filing on Friday, Mueller rebuked that assertion.

“Nothing about the way the interview was arranged or conducted caused the defendant to make false statements to the FBI on January 24,” Mueller wrote, asking a federal judge to reject Flynn’s attempt to “minimize the seriousness of those false statements to the FBI.”

An NBC News/Wall Street Journal poll released Sunday morning found that 62 percent of respondents believe Trump has been untruthful about the Russia investigation, while 50 percent said the investigation has given them at least some doubts about his presidency. Forty-four percent said it has not given them additional doubts.

Trump forced Sessions to resign last month in a widely anticipated move.

Trump had for months publicly derided Sessions over his decision to recuse himself from overseeing the Mueller investigation, and at one point suggested he only appointed him to the Cabinet post because the former Alabama senator had shown loyalty during the 2016 campaign.

The president appointed Matthew Whitaker to take over as acting attorney general, and has since nominated former Attorney General William Barr for the post full time.

The pick is likely to be highly scrutinized by Democrats, many of whom viewed Sessions's ouster as a threat to the special counsel's investigation.