Senate Majority Leader Mitch McConnell (R-Ky.) talks to reporters at the U.S. Capitol on Tuesday. (Chip Somodevilla/Getty Images)

LAST WEEK, President Trump gave the clearest indications to date of wanting to fire special counsel Robert S. Mueller III or his Justice Department bosses in an effort to foil his investigation. This week, Senate Majority Leader Mitch McConnell (R-Ky.) responded by refusing to allow the Senate to consider legislation to protect Mr. Mueller’s tenure.

“There’s no indication that Mueller’s going to be fired,” Mr. McConnell explained to Fox News’s Neil Cavuto. “I don’t think the president’s going to do that.” We hope Mr. McConnell knows something the rest of the country does not. But what matters is not the majority leader’s hunch about the president’s whims; what matters is whether Congress will make a statement that would make a firing less likely.

All outward appearances are that Mr. Mueller has never been in greater peril. The president continues to seethe about a search warrant served on Michael Cohen, his personal lawyer. Mr. Trump mused on live television last week about terminating Mr. Mueller, and his press secretary insisted that the president has the power to dismiss the special counsel. CBS News reported that Mr. Trump encouraged a TV commentator to argue on air for canning Mr. Mueller. These events persuaded many senators, Democrats and Republicans, to express concern about Mr. Mueller’s fate. Sen. Charles E. Grassley (R-Iowa), chairman of the Judiciary Committee, set aside time to consider a bipartisan bill meant to protect the special counsel from improper dismissal.

Now would be the time for lawmakers to show they back the special counsel’s legitimate investigation. Instead, Mr. McConnell insisted that the full Senate would not entertain the bill Mr. Grassley put on the Judiciary Committee’s calendar, an act that sends the opposite signal.

Mr. McConnell argued that passing a bill insulating the special counsel from the White House would be futile because the president would veto it. Given the high stakes for the rule of law and the legislature’s role in checking the executive, one would hope a veto-proof majority favoring this bill existed in Congress. Yet even passing the bill by simple majority would signal lawmakers’ seriousness and warn Mr. Trump that he might face substantial congressional blowback if he was to fire Mr. Mueller. I t would be far better for Congress to deter Mr. Trump from making a rash move, rather than wait to react after he has inflamed passions and further eroded trust in law enforcement.