Plenty of Republicans in Congress have been warning President Donald Trump not to try to fire special counsel Robert Mueller, and he’s reportedly been pulled back from the brink of doing so at least a couple of times. But after the raids this week on the home, office and hotel room of his personal attorney, Mr. Trump has increasingly been focusing his ire on another official whose firing might not cause such a political firestorm: Deputy Attorney General Rod J. Rosenstein. Republican officials haven’t said whether they would consider his firing to be a red line like they have for Mr. Mueller, but it should be. His role in upholding the rule of law may at this point be even more crucial than that of the special counsel.

President Trump is reportedly considering firing the deputy A.G., who personally signed off on the Justice Department’s decision to seek search warrants for attorney Michael Cohen’s records on assorted Trump-related matters, including a $130,000 hush-money payment to adult film actress Stormy Daniels. On Wednesday morning, Mr. Trump lumped Messrs. Rosenstein and Mueller together in a tweet alleging political conflicts of interest for both in the Russia investigation, claiming they are “Democrat loyalists, or people that worked for Obama.

(The whole Twitter thread, rather astonishingly, suggests that the pair are responsible for Russian President Vladimir Putin’s intransigence in Syria and his threats to shoot down any missiles the U.S. fires into that country in response to a recent chemical weapons attack there.)

Mr. Rosenstein did, of course, work for the Obama administration as the U.S. attorney for Maryland, but he was appointed to that post by George W. Bush. And if he did that job with an aim toward pleasing the Democrats, it would probably be news to the officials in that party he prosecuted or for whom he caused huge political headaches. Among the elected Democrats he sent to prison was former Prince George’s County Executive Jack Johnson, and the massive investigation his office oversaw into corruption at the Baltimore City Detention Center proved problematic for former Gov. Martin O’Malley, who was just about to launch a campaign for president on his reputation as a proficient manager. One of his last acts in Maryland was the indictment of members of the Baltimore Police Department’s Gun Trace Task Force, another case that put the city’s Democratic leadership in a bad light.

The Democratic powers-that-be in Maryland didn’t push for President Obama to replace him. That wasn’t because he went easy on Democrats but because he exuded not a whiff of political motivation during his long tenure here. He did his job, and he did it well.

By all appearances, his approach has not changed since taking the post as second in command at the Department of Justice under President Trump. We questioned his authorship of a memo making the case for President Trump to fire James Comey — not because his argument that the then-FBI director had acted wrongly in his public statements about the investigation into Hillary Clinton’s email but because of the broader context of the bureau’s investigation into possible Russian collusion with the Trump campaign. Mr. Rosenstein promptly corrected that deficiency with the appointment of Mr. Mueller, himself a highly respected former FBI director (and a Republican). Since then, he has been the crucial bulwark in keeping the Mueller probe free of interference.

Two things may now put him in additional peril. The first is the possibility that Mr. Trump could see firing Mr. Rosenstein as a less politically costly way to hem Mr. Mueller in; because the special counsel reports to the deputy attorney general (given A.G. Jeff Sessions’ recusal from all Russia investigation-related matters), the person in that job can limit the scope of the probe in ways that neuter it without killing it outright. Steve Bannon is reportedly pushing a plan along those lines. The second reason Mr. Rosenstein may be at risk is that the legal peril for Mr. Trump and his associates has now spread beyond the Mueller inquiry. It wasn’t Mr. Mueller’s investigators who seized Mr. Cohen’s records but the U.S. attorney’s office for the Southern District of New York — acting on the approval of Mr. Rosenstein (among others).

Sen. Chuck Schumer, the New York Democrat, said this week that firing Mr. Rosenstein would be just as bad as firing Mr. Mueller. Some of Mr. Rosenstein’s fellow Republicans need to make the same point.