Former Attorney General Eric Holder Eric Himpton HolderThe Hill's Campaign Report: Trump's rally risk | Biden ramps up legal team | Biden hits Trump over climate policy Biden campaign forming 'special litigation' team ahead of possible voting battle Pompeo, Engel poised for battle in contempt proceedings MORE on Sunday called for Americans to hold peaceful mass protests if 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 fires special counsel Robert Mueller amid the ongoing investigation into Russia's election interference.

"ABSOLUTE RED LINE: the firing of Bob Mueller or crippling the special counsel's office. If removed or meaningfully tampered with, there must be mass, popular, peaceful support of both," tweeted Holder, who served as the top prosecutor in the Obama administration.

ABSOLUTE RED LINE: the firing of Bob Mueller or crippling the special counsel’s office. If removed or meaningfully tampered with, there must be mass, popular, peaceful support of both. The American people must be seen and heard - they will ultimately be determinative. — Eric Holder (@EricHolder) December 17, 2017

Holder has repeatedly warned that the dismissal of Mueller from the federal probe would "not be tolerated," telling congressional Republicans last week to not make any moves to fire Mueller.

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“Speaking on behalf of the vast majority of the American people, Republicans in Congress be forewarned: any attempt to remove Bob Mueller will not be tolerated,” Holder tweeted on Thursday.

“These are BS attacks on him/his staff that are blatantly political-designed to hide the real wrongdoing. Country not party,” he continued.

Holder's defense of Mueller and his ongoing probe come after rumors swirled last week that Trump planned to fire the special counsel, who is leading the federal probe investigating whether Trump campaign aides colluded with Moscow during the 2016 presidential election.

A White House lawyer handling Trump's response to Mueller's investigation has denied the dismissal buzz.

Fox News and conservatives in recent weeks have stepped up their attacks against Mueller, prompting widespread bipartisan support from lawmakers who say Mueller's investigation should be able to run its course.

The president's allies have recently seized on reports that Mueller removed FBI agent Peter Strzok from his team after an investigation revealed that the veteran agent sent anti-Trump text messages during the 2016 campaign. They argue that Strzok's messages are proof that Mueller's team is leading a politically biased investigation.