Virginia Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, delivered a powerful warning to the Trump administration and its Republican allies this week, when he declared on the Senate floor that a move by the president to fire special counsel Robert Mueller or to undermine Mueller’s inquiry would cross one of the “red lines” that Congress must maintain.

“Any attempt by this President to remove special counsel Mueller from his position or to pardon key witnesses in any effort to shield them from accountability or shut down the investigation would be a gross abuse of power and a flagrant violation of executive branch responsibilities and authorities. These truly are red lines and [Congress] simply cannot allow them to be crossed,” explained Warner. “Congress must make clear to the President that firing the special counsel or interfering with his investigation by issuing pardons of essential witnesses is unacceptable and would have immediate and significant consequences.”

Warner’s right. Though Trump aides deny that the president is angling to shut down Mueller’s investigation into Russian meddling with the 2016 presidential election—and a host of other issues that cut close to the president and his inner circle—there can be no question that the Trump team and its media allies have launched a campaign to discredit the special counsel. This has stirred speculation on Capitol Hill that Mueller and his investigation are being attacked in order to clear the way for a firing. So high marks to Warner, and to others who have raised concerns. It is vital to get ahead of these threats.

But if congressional Democrats—and those responsible Republicans who might be inclined to get on the right side of history—really want to put Trump on notice, they must take the next step and explain what the “immediate and significant consequences” would be. To do this, Democrats need to start using the word “impeachment.” Impeachment begins in the House, not the Senate, so Mark Warner has no responsibility to propose the articles indicting Trump. But members of the House and Senate have a duty to make it clear—to Trump and to the American people—that there is a specific and appropriate answer to a gross abuse of power.

Top congressional Democrats tend to cautious about doing this. But that caution sends a mixed signal to the president and his minions. And to the hundreds of thousands of Americans who are prepared to respond—as part of mobilizations such as the “Nobody is Above the Law—Mueller Firing Rapid Response” being promoted by Move On and other groups.

Yes, impeachments are challenging—practically and politically. House Speaker Paul Ryan, R-Wisconsin, and the Trump-aligned members of the House and Senate would undoubtedly erect roadblocks.

But when a president creates a constitutional crisis, the difficulty of responding does not change the fact that the right response is the application of the constitutional remedy that the founders outlined.

When the Constitutional Convention of 1787 defined 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 authority 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?”

If Trump commits the most extensive injustice, Democrats (and responsible Republicans) should make it clear that his abuses of power will be met with the power of impeachment.