Republican senators who have warned the White House for months not to interfere with 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’s investigation are coming under new pressure given President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s latest demands for the Department of Justice.

Republican senators believe that if Trump fired Mueller or obviously interfered in his investigation, it would backfire politically and potentially hurt their chances of keeping control of Congress.

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They want to avoid an all-out war between Trump and Justice that might draw parallels to the Nixon-era “Saturday Night Massacre,” when senior department officials resigned instead of complying with orders to fire a special counsel.

“I don’t like seeing demands made, especially about political issues,” Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) said Monday in reaction to Trump’s demand that the Department of Justice investigate the FBI’s use of an informant during its investigation of the 2016 election.

“I don’t think it’s a good idea. I think it’s better to allow the Department of Justice to make its own decisions and not comment,” said Sen. Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-Maine).

The possibility of a standoff between the White House and Justice that would evoke the firing of Watergate-era special prosecutor Archibald Cox faded for the time being on Monday when officials said the president had agreed to 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’s decision to refer the matter to the department’s inspector general.

White House press secretary Sarah Huckabee Sanders said in a statement that Justice, FBI and intelligence officials reached an agreement with Trump during a meeting Monday afternoon that the inspector general should “expand its current investigation to include any irregularities with the Federal Bureau of Investigation’s or the Department of Justice’s tactics concerning the Trump Campaign.”

That’s good news for GOP senators worried about getting between Trump and the Justice Department. But it’s far from clear how long the detente will last.

“It is important for members of Congress to protect the integrity of the investigation into Trump,” said Darrell West, director of governance studies at the Brookings Institution.

“They cannot allow the president to compromise the investigation or keep it from running its full course.”

Sen. John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (Texas), the No. 2 Republican in the Senate, said Rosenstein had made the right call — but defended Trump at the same time for wanting to make sure Obama-era officials weren’t taking improper actions against him.

“I thought the president’s point was an important one,” he told reporters. “It is unthinkable ... that the leader of the opposing political party would have the authority to use the FBI to place an informant, potentially, in a political campaign of his opponents. And, you know, we need to know more about that.”

“I think Rod Rosenstein ... made the right decision,” he added.

Trump allies for days have said there may have been an informant embedded in the Trump campaign. No evidence has suggested this was a reality.

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.), a frequent critic of Trump’s, also said Rosenstein “did what he should have done.”

“That’s where it belongs,” he said of the decision to let the inspector general review it.

Flake said it would be inappropriate for the department to “allow the president to dictate who’s investigated and who isn’t.”

“That’s just not the president’s call,” he said. “It never has been.”

Legal experts say Trump’s weekend attacks took his battle with the law enforcement agency to a new level.

“He’s certainly crossing a line,” said Harry Litman, a former U.S. attorney and deputy assistant attorney general. “It’s foreboding and deeply inappropriate for a president to be calling for this kind of criminal investigation of his perceived enemies.”

“The second order problem here is these are not political enemies, this is the FBI. There’s not an Obama FBI and a Trump FBI,” he added. “It is the Federal Bureau of Investigation and their watchword is they don’t initiate investigations for political reasons.”

Litman noted that the FBI reportedly used an informant to solicit information from two Trump campaign officials, 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 and Carter Page, after receiving evidence that they had suspicious Russian contacts.

The FBI launched a counterintelligence investigation into Trump’s campaign after Australian officials alerted U.S. intelligence that Papadopoulos had bragged during a night of drinking at a London bar that Russia had acquired information that would hurt Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE’s chances in the 2016 presidential election.

Former federal prosecutors say the use of an informant to acquire information is routine and doesn’t even require a search warrant.

They say that Trump’s characterization of the use of the informant as a surveillance operation is misleading.

“It’s completely false to say there was an FBI person planted there,” said Litman. “To use a confidential informant doesn’t even require a search warrant. That’s something that every law enforcement agency does every day.”

Matthew Orwig, a former U.S. attorney for the Eastern District of Texas, said there’s “nothing improper” about using an informant.

“This is all going to depend on what exactly the nature of the informant and the purpose of the informant,” he said, but added “the technique itself is absolutely accepted.”

“There’s certainly valid concerns and there should always be a lot of respect for the independence of the department,” Orwig added. “Should there be a heightened awareness about something that interferes with the independence of the Department of Justice or that interferes with the role of law enforcement?”

“Skepticism is valid,” he said.

Jordain Carney contributed.