Lawmakers on Sunday rallied to the defense of special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE after concerns were raised over his job security following the abrupt firing of FBI Deputy Director Andrew McCabe Andrew George McCabeGraham: Comey to testify about FBI's Russia probe, Mueller declined invitation Barr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' GOP votes to authorize subpoenas, depositions in Obama-era probe MORE.

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 announced on Friday that he fired the FBI’s No. 2 official in a move that roiled Washington, D.C., and spurred a series of tweets from President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE denouncing Mueller, McCabe and former FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE.

McCabe said his firing was an attempt to undermine the Mueller investigation into Russia's election interference and possible collusion between members of Trump's campaign and Moscow. The president targeted Mueller's investigation in a series of tweets over the weekend, further alarming many lawmakers.

Democrats on Sunday were calling for proactive measures to protect Mueller and his investigation. Republicans insisted Trump has no intention of firing the special counsel, although the White House also acknowledged Trump is "frustrated."

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“Give him the time, the resources, the independence to do his job,” House Oversight and Government Reform Committee Chairman Trey Gowdy Harold (Trey) Watson GowdySunday shows preview: Election integrity dominates as Nov. 3 nears Tim Scott invokes Breonna Taylor, George Floyd in Trump convention speech Sunday shows preview: Republicans gear up for national convention, USPS debate continues in Washington MORE (R-S.C.) told “Fox News Sunday” of Mueller.

“Let it play out its course. If you’ve done nothing wrong, you should want the investigation to be as fulsome and thorough as possible,” he added of Trump.

Trump has repeatedly denounced the Russia investigation as a "witch hunt." But the president’s most recent attacks on Mueller come after the special counsel reportedly subpoenaed the Trump Organization and, according to another report, provided Trump’s legal team with a series of questions leading up to a possible interview with Trump. McCabe has also provided the special counsel with memos describing his contacts with Trump, according to multiple reports.

While Trump's team has quietly cooperated with Mueller over the past weeks, the president over the weekend seemed to unleash his frustration with the investigation.

“The Mueller probe should never have been started in that there was no collusion and there was no crime,” Trump wrote on Twitter Saturday. He went on to criticize on Sunday the members of Mueller's team as "13 hardened Democrats, some big Crooked Hillary [Clinton] supporters, and Zero Republicans." He also lashed out at reports McCabe had memos on their interaction on Sunday, tweeting that McCabe “never took notes when he was with me.”

Democrats have long expressed concerns over Trump’s attacks on the special counsel, and lawmakers in both parties have put forward legislation that would enshrine legal protections for Mueller.

Rep. Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.), the ranking member of the House Intelligence Committee, called on his colleagues to “speak out now” and not wait for the possible “crisis” of Trump firing Mueller.

“I think, George [Stephanopoulos], you just pointed to the single most important development of the week and that is at the same time it's revealed that the special counsel is looking at business records of the Trump Organization, and I've always thought the money laundering issue was the most serious, you have the president through his lawyer trying to shut down the Mueller investigation and speaking out against special counsel,” Schiff told ABC’s "This Week."

Trump's personal attorney John Dowd on Saturday called on Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE to "bring an end" to Mueller's investigation, prompting increased concern from Trump critics that the president was trying to influence the Justice Department over the probe.

Such a move would be “a massive red line that can’t be crossed,” according to Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.).

“So, I hope that that's the case [for most Republicans]. And I would just hope that enough people would prevail on the president now, don't go there. Don't go there,” Flake told CNN.

Senate Minority Whip Dick Durbin Richard (Dick) Joseph DurbinMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled Senate Republicans signal openness to working with Biden Top GOP senator calls for Biden to release list of possible Supreme Court picks MORE (D-Ill.) echoed those concerns, saying it would be a “constitutional crisis” should Trump stop the special counsel’s probe.

And Sen. Lindsey Graham Lindsey Olin GrahamMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-S.C.) said Trump dismissing Mueller “would be the beginning of the end of his presidency.”

“I pledge to the American people, as a Republican, to make sure that Mr. Mueller can continue to do his job without any interference,” Graham told CNN’s “State of the Union.” “I think he's doing a good job.”

Both Graham and Flake — who are members of the Senate Judiciary Committee, which is leading its own Russia probe — suggested the committee would look into McCabe's firing.

Sen. James Lankford James Paul LankfordMcConnell works to lock down GOP votes for coronavirus bill Charities scramble to plug revenue holes during pandemic Warren calls for Postal Service board members to fire DeJoy or resign MORE (R-Okla.), who sits on the Senate Intelligence Committee, said McCabe’s firing is “not surprising in many ways" and suggested he deserved it, due to the contents of an internal review. The full report has not been released so the evidence that got McCabe fired is not fully known.

But the Oklahoma lawmaker, like most of his colleagues on both sides of the aisle, said Mueller should be allowed to complete his investigation.

“I would say the best thing the special council can do is to finish the investigation, gather all the information that’s needed, come to a conclusion so the American people can make their own decisions,” Lankford told ABC's “This Week.”

Trump has grown frustrated with the length of the Russia probe, White House legislative affairs director Marc Short told CBS’s “Face the Nation.” But Short said that no White House officials are recommending the administration stop cooperating with Mueller’s investigation.

-Update 1:46 p.m.