The only thing that Donald Trump’s first speech to the nation from the Oval Office proved is that Trump is no good at making speeches to the nation from the Oval Office. The president failed on Tuesday evening to deliver an even minimally credible case to support his claim that there is “a growing humanitarian and security crisis at our southern border.” Nothing that he said in a rambling eight-minute recitation of talking points and applause lines from campaign rallies could justify his use of emergency powers to build an unnecessary and unworkable border wall.

These are the vital takeaways from Trump’s speech, as the president and his aides have in recent days been suggesting that he might declare a national emergency as part of a scheme to “secure the border.”

Presidents have limited authority to declare national emergencies when there are crises so severe that immediate action is necessary. But presidents cannot abuse this authority; they must operate within boundaries established by judicial precedent and the 1976 National Emergencies Act.

These actual facts must frame the debate about this president’s fact-free assertion that he might invoke emergency powers to justify the expenditure of billions of dollars to build a border wall that has not been authorized by  Congress. And they create the context in which Tuesday’s speech must be assessed.

Every president is duty-bound to make a compelling case for the use of emergency powers. That was what made Trump’s extraordinary address to the nation Tuesday evening such a big deal.

And such a disaster for his crumbling presidency.

Trump wanted to portray his border-security agenda as “just common sense.” But there was nothing sensible about the squinting president’s attempt to read from the “how-much-more-American-blood-must-we-shed” script that his fearmongering speechwriters had crafted.

Claiming that he was determined to end a “cycle of human suffering,” Trump offered up a stew of outright lies and false premises. He tried to suggest that he sympathized with immigrants, while at the same time portraying them as violent criminals. He repeated his absurd promise that Mexico will pay for the wall, with the footnote that this will now happen “indirectly.” He asserted that immigrants take jobs from American workers, at a time when American employers say they struggle to fill positions. He tried to blame congressional Democrats for a government shutdown, after telling Chuck Schumer and Nancy Pelosi less than a month ago that “I will take the mantle. I will be the one to shut it down—I’m not going to blame you for it.”

Trump’s address was precisely the “word salad of nonsense” that the Rev. Jesse Jackson predicted it would be. It did nothing to change the reality that, as the civil-rights leader noted, “There is no crisis at the southern border. No caravans, trying to sneak in; no terrorists coming across in mass numbers. The greatest number of undocumented immigrants in the United States are those who have overstayed their visas after coming here legally by plane.” And it did nothing to justify what Jackson decried as Trump’s “phony emergency.”

Since the president failed to make the case that was required of him, Congress must be prepared to check and balance Trump before his infamy becomes a disaster for the nation.

“If the president follows through on the threat to declare a state of emergency simply to circumvent the legislative branch and build a wall on the Mexican border,” says Common Cause president Karen Hobert Flynn, “then Congress must act swiftly and decisively to check the abuse utilizing the National Emergencies Act, which was enacted in 1976 as a post-Watergate reform to reassert Congress’s constitutional role in checking and safeguarding against authoritarian abuses of power.”

That will not be easy in the divided city of Washington. But it is necessary. The declaration of an emergency when no emergency exists, especially when it is done with the purpose of diverting money to a project that is unauthorized, must always be met with an urgent response. This response needs to make clear that abuses of power cannot be permitted without a fight.

Democrats should be prepared to force the issue. Doing so will signal that responsible elected officials, hopefully from both parties, do not accept the president’s outrageous—and dangerous—claims regarding immigrants. Doing so will also indicate that members of Congress are prepared to do battle on behalf of the Constitution. Even if efforts to check and balance the president fail in the short term, a challenge to Trump on this vital issue allows House Democrats to claim the moral and political high ground as they assert their new majority status.

Trump, for his part, has no ground to stand on.

Before he demanded television time in a desperate attempt to justify his increasingly erratic and irresponsible behavior, the president had already been called out for lying in his pronouncements regarding his wall. Yet the lies have not worked. Trump’s argument that a border “crisis” demands urgent action has been so uninspiring that—after months of his advancing various theories in this regard—a Hill-HarrisX poll released Monday found that less than one-third of registered voters support the government shutdown Trump has engineered in hopes of forcing Congress to fund the wall.

According to The Hill, “70 percent of registered voters want Trump and Congress to reach some sort of compromise, compared to just 30 percent who say that sticking to principles is more important than ending the partial shutdown.”

Those devastating numbers explain why Trump was so determined to present his televised claim that a crisis exists. But president did not make a credible claim, and he certainly did not justify a power grab.

Even if Trump had not bumbled so badly on Tuesday night, however, he faced legal jeopardy. The New York Times has argued that any attempt by Trump to declare a national emergency is “almost certain to invite a court battle.” As the Times observes, “Although presidents have sometimes claimed that the Constitution gives them inherent powers to act beyond ordinary legal limits in an exigency, those claims tend to fare poorly when challenged in court.”

But Congress need not wait for court battles to play out. Indeed, Congress has a responsibility to use its authority under the National Emergencies Act to block Trump. If and when Trump declares an emergency, majority votes in the House and Senate can pass a resolution terminating the emergency. The president may veto that resolution, and then two-thirds votes of both chambers would be required to override the veto.

That’s daunting. But no more daunting than many of the processes of accountability that are outlined in the Constitution.

It may be that dead-ender Republicans will side with the president if he makes his “emergency” move. So be it. Democrats and responsible Republicans should expend every effort to force debates and votes on this issue. If majorities in the House and Senate are on the record in opposition to this abuse of emergency powers, that would be a constitutionally consequential intervention.

What is at stake is not merely the wall, nor even the shutdown fight. This is about a much more important principle. The Constitution of the United States, to which members of Congress swore their allegiance last week, establishes separated powers. But those powers only have meaning if Congress checks and balances the executive when the executive acts as an imperial president.

“The Republican majorities in the last Congress refused to act to curb a variety of abuses by President Trump and that has only served to further embolden him,” says Common Cause’s Flynn. “If President Trump declares a national-emergency in order to build a wall, the new Congress must act immediately, using the checks and balances written into our Constitution to prevent the president’s abuse of power.”