Sen. Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE (I-Vt.) said on Sunday that he is "very concerned" that President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE could move to fire special counsel Robert Mueller amid the investigation into Russia's election meddling.

"To attack his integrity in order to protect the president is unacceptable. Furthermore, it is equally unacceptable for the president or any of his advisers to be thinking about granting pardons to those people who pled guilty," he told CNN's Jake Tapper on "State of the Union."

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Trump's former campaign chairman, 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, and Manafort's business associate, Richard Gates, were both charged with conspiracy against the U.S. in October, while former campaign aide 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 pleaded guilty to making false statements to the FBI.

The president's former national security adviser, Michael Flynn, pleaded guilty earlier this month to lying to the FBI about his foreign contacts.

Sanders's comments come as speculation swirls over whether the president will fire Mueller in the middle of the federal probe into alleged ties between the Trump campaign and Russia's election meddling.

The president and the White House have dismissed speculation that Mueller would be fired, but Trump has ripped the probe, calling it a "witch hunt."

Republicans have also criticized the probe, citing text messages that were critical of Trump from FBI agent Peter Strzok during the campaign.

Strzok has since been taken off of Mueller's probe.

A number of Republican lawmakers, including Rep. Trey Gowdy (R-S.C.), have urged the president not to fire Mueller, however.

“He’s the best hope for giving us a product that the largest number of Americans can accept as credible,” Gowdy said.