The Justice Department this week threw its weight behind a simple principle: Inciting an insurrection doesn’t fall within a member of Congress’s duties. Obvious as the declaration may seem, Attorney General Merrick Garland’s support for it is essential to a functioning democracy.

“Are you willing to do what it takes to fight for America?” Rep. Mo Brooks (R-Ala.) enjoined supporters of then-President Donald Trump at a Jan. 6 rally preceding the storming of the U.S. Capitol. Mr. Brooks invoked bloodshed and death, and exhorted that participants not let the Democratic Party “steal from us our God-given right to control our nation’s destiny.” Now he’s arguing that he spoke these words as part of his professional obligations and therefore may enlist the government to step in as defendant in a lawsuit that Rep. Eric Swalwell (D-Calif.) filed against him and others, including Mr. Trump.

The Justice Department, in a court filing on Tuesday, argues this is absurd. The department is right. Substantial protections are afforded to elected officials for statements made in the context of their jobs, according to interpretations by both courts and this very Justice Department. Yet there must be limits on these protections, and if Mr. Brooks’s behavior doesn’t exceed them, it is hard to imagine any egregious act for which an official couldn’t claim to be automatically exonerated.

The administration’s primary contention is somewhat technical, homing in on the distinction between a representative’s professional and political activities, including campaigning or electioneering. It argues that Mr. Brooks’s words at the Jan. 6 rally fall into the latter category, which is not protected within the scope of a lawmaker’s employment.

Regardless, the department argues, advocating the overthrow of one’s employer is categorically inexcusable as within the scope of one’s job. That’s true for a representative, and it’s true for any federal employee — including the president. It is this part of the brief that could be most significant going forward.

Mr. Garland has spent his tenure so far picking through a political and institutional thicket: tasked on the one hand with holding the previous administration accountable for any legal wrongs and on the other with ensuring that prosecutorial decisions aren’t based on partisan considerations. Too much concern about this second point, his critics on the left have warned, could result in his department overcorrecting by not pursuing prosecutable cases, or by defending indefensible ones, in an effort to avoid any appearance of bias.

The attorney general has signaled he doesn’t plan to fall into that trap. Mr. Trump, who insists he has “complete immunity” for his role in the winter’s riot, may want to worry about his own footing.

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