President Donald Trump’s plan to use his national emergency powers to order the military to build a wall on the U.S.-Mexico border is a clear and utter fraud. How do we know this? His own administration tells us so.

Late last month, Director of National Intelligence Dan Coats presented the latest version of the Worldwide Threat Assessment of the U.S. Intelligence Community to the Senate Select Committee on Intelligence. It runs to 42 pages, describing threats from cyber attacks, influence operations around elections, terrorism, proliferation of weapons of mass destruction, drug trafficking and other global trends that could put America’s citizens and interests at risk. What about the migrants from Central America seeking to cross the southern U.S. border? It mentions them, on page 41, in a paragraph noting the stepped-up efforts of their countries of origin and Mexico to stop them.

The situation at the border presents an emergency, all right — a political emergency for President Trump. He has spent the last three years promising to build a wall as away to stoke xenophobia among his political base, yet he was unable to persuade Congress to authorize it when his party controlled both chambers, and he certainly can’t now that Democrats have a majority in the House. He has learned that shutting down the government doesn’t work either as a negotiating or a political tactic, but he also can’t give up on the wall without enraging his base. It’s a problem, and one entirely of his own making.

The Constitution gives Congress, not the president, the power to raise funds and appropriate them. Congress has allowed a narrow exception — a president has some authority to appropriate funds in ways Congress did not specifically authorize during a national emergency. Congress has unequivocally rejected the idea that the situation at the border requires building Mr. Trump’s wall. His declaration is a matter of convenience for him, not of urgent action to protect the nation.

The matter will certainly be taken to court by outside groups and possibly the Democratic House majority. They have a strong case, at least for parts of the funding Mr. Trump reportedly intends to use. For example, the largest pot of money potentially available to him is in the Department of Defense, but using it even under an emergency is only allowed when the crisis at hand requires the use of the armed forces. Since border security is handled by the Department of Homeland Security, and since Mr. Trump has heretofore proposed building the wall using civilians, it patently does not.

But the more important question is how Congress will react. The law allows the House and Senate to pass a resolution rejecting the declaration of an emergency, and it prevents the Republican leadership in the Senate from blocking a vote on the matter if it is passed by the House. Several Republican senators have already voiced objections to the president’s action, so it appears likely that such a measure would pass.

But President Trump would have the opportunity to veto it — and surely he would, for the drama that would cause if no other reason — and Congress could only override him with a two-thirds vote in both chambers. At that point, the Republican Party would have to consider what is more important: giving this president a win on building a wall the majority of Americans don’t want or maintaining a semblance of checks and balances. If they have even the slightest regard for the Constitution — not to mention self-interest, given the possibility that a future Democratic president could declare an emergency to deal with something that actually is an emergency, like climate change — they’ll choose the latter.

We know what they should do. So do they. But sadly, we also know they won’t do it. Mr. Trump may get his wall, but for Republicans, it will come at a tremendous cost.