President Trump stops to speak to reporters as he walks to board Marine One. (Jabin Botsford/The Washington Post)

PRESIDENT TRUMP’S attacks on independent law enforcement — and on special counsel Robert S. Mueller III in particular — have become more aggressive in recent days. That makes it all the more urgent that House Speaker Paul D. Ryan (R-Wis.) and Senate Majority Leader Mitch McConnell (R-Ky.) act to deter Mr. Trump from a catastrophic decision to fire Mr. Mueller.

Breaking from his lawyers’ advice not to talk about the special counsel, Mr. Trump tweeted on Saturday, “The Mueller probe should never have been started in that there was no collusion and there was no crime.” On Sunday, he falsely claimed that there were no Republicans on Mr. Mueller’s investigative staff. On Monday, he inaccurately called the probe “a total WITCH HUNT with massive conflicts of interest!”

It is tempting to ignore the president’s nonsense rather than to dignify it with response. But the president, with ignoble help from Republicans on the House Intelligence Committee, is planting seeds of doubt about Mr. Mueller and the FBI as their investigation apparently intensifies on his businesses and behavior. It is becoming increasingly imaginable that he would attempt to short-circuit the probe.

Rep. Trey Gowdy (R-S.C.) warned on Sunday that the president would endure “a very, very long, bad 2018” if he did so. Sen. Lindsey O. Graham (R-S.C.) said firing Mr. Mueller would be “the beginning of the end of his presidency.” But Republican leaders have given Mr. Trump insufficient reason to heed Mr. Graham’s warning. Through a spokeswoman, Mr. Ryan managed a wan “Mr. Mueller and his team should be able to do their job,” while Mr. McConnell said nothing and House Majority Whip Steve Scalise (R-La.) echoed Mr. Trump’s baseless accusations that the Mueller investigation has “credibility concerns.”

Those cannot be the last words from a self- ­respecting branch of government. Congress should pass a bill seeking to protect Mr. Mueller from being fired without cause. Legislation along these lines exists but is languishing for lack of support among Republicans. Passing it would send a clear, public message that congressional leaders have so far declined to convey: Firing Mr. Mueller would elicit a substantial real-world reaction that would severely harm the White House.

The president treats government as purely a tribal battle between one side and the other and expects public servants to put loyalty to him before loyalty to country or the rule of law. Republican congressional leaders should not remain complicit in this debasement.