Walter Shaub, the former director of the US Office of Government Ethics who now serves as a senior director focusing on ethics issues for the Campaign Legal Center, wants the Trump administration, its surrogates, and its allies to back off from what the center refers to as “their attempt to undermine the investigation led by special counsel Robert Mueller.”

Noting efforts by the president’s lawyer, Jay Sekulow, and others to “muddy the waters and impede Mueller’s investigation,” the center issued a statement Friday from Shaub, in which he said:

The coordinated effort by President Trump and his surrogates to discredit the Mueller investigation raises serious alarms. Rather than making themselves complicit in this assault on the rule of law, Members of Congress should send a clear message to the President that firing Mueller is a red line he must not cross.

Shaub is right about the red line. If Trump fires Mueller, as many now speculate is possible, the United States will find itself in a constitutional crisis—where the executive branch rejects scrutiny, checks and balances, and the rule of law in order to protect itself from accountability. The expert on government rights and wrongs is so concerned that—after a Trump-aligned member of the House, Florida Congressman Matt Gaetz, appeared on CNN and announced: “I call on my Republican colleagues to join me in the firing of Bob Mueller”—Shaub wrote: “Make a plan folks. Be ready to take to the streets. This is an attack on our Republic.”

Taking it to the streets is always a good idea when the oligarchs and plutocrats spin out of control. That’s why the founding generation of the American experiment established First Amendment rights to assemble and to petition for the redress of grievances. But it is important, now, before a potential crisis becomes a real one, to signal what that reaction will be.

Citizens need to know what they will demand when they assemble and petition for the redress of grievances.

So what’s the right demand? If a president attacks the Republic, there is one correct and necessary response: impeachment.

When the Constitutional Convention of 1787 outlined the impeachment power, there was a good deal of clarity as regards when and how it should be employed. It was to serve as a check and balance on the executive branch in general, and on president’s in particular. “No point is of more importance than that the right of impeachment should be continued,” argued George Mason. This check on presidential abuses of power provided an answer to the questions that vexed Mason: “Shall any man be above Justice? Above all shall that man be above it, who can commit the most extensive injustice?”

Some members of Congress recognize their duty in these times to answer those questions with a clear commitment to hold this president to account. Congressmen such as Tennessee Democrat Steve Cohen, the ranking Democrat on the Constitution subcommittee of the House Judiciary Committee, and California Democrat Brad Sherman, have proposing impeachment resolutions—with an appropriate focus on concerns about presidential obstruction of justice, and with an appropriate sense of urgency. But when Texas Democrat Al Green raised the issue of impeachment in the House this month, his motion was tabled on a 364-58 vote.

Top Democrats joined House Republicans in voting to table Green’s motion, with House minority leader Nancy Pelosi of California and Democratic whip Steny Hoyer of Maryland releasing a joint statement that read:

Legitimate questions have been raised about his fitness to lead this nation. Right now, Congressional committees continue to be deeply engaged in investigations into the President’s actions both before and after his inauguration. The special counsel’s investigation is moving forward as well, and those inquiries should be allowed to continue. Now is not the time to consider articles of impeachment.

Supporters of a robust system of checks and balances were disappointed by the caution Pelosi and Hoyer displayed. But the attention the Democratic leaders paid to Mueller’s inquiry establishes a red line of the sort Shaub describes.

Pelosi and Hoyer have always been cautious about the “I” word. But if Trump fires Robert Mueller, there will be no more room for caution. And one of the best ways to defend the Mueller inquiry is for House Democrats to make it clear at this point that—while they are, indeed, a minority in the House and Senate, and while they surely face the obstacle of House Speaker Paul Ryan’s partisanship-over-principle approach to governing—their response to the firing of Mueller would be an absolute and unequivocal demand for the impeachment of Donald Trump.