Moments after President Trump announced his border emergency declaration in a Rose Garden speech, Democrats pounced, salivating over their own future national emergencies they would someday be able to declare using similar logic.

Sen. Chris Murphy, D-Conn., touted an assault weapons ban by executive action. Rep. Ilhan Omar, D-Minn., alluded to the Green New Deal. The very thought should horrify everyone: the sheer possibility of mass gun confiscation or the unilateral imposition of a $93 trillion program which would certainly nationalize nearly half of the American economy without changing global temperatures.

Unfortunately, Congress abdicated its authority and constitutional duty to define and restrict the president's national emergency powers long ago. The National Emergencies Act, passed by Congress after Watergate, failed to craft clear legal bounds of what does and doesn't constitute a national emergency, forcing the American people to rely on the mercy of precedents, not constitutional lawmaking.

Although the amount of funds Trump has announced he'll use executive authority to appropriate for border security is much greater than the specific portion delineated in his national emergency declaration, the rationale seems to meet up with the law in question. But that law's constitutionality is dubious. If Congress and the courts fail to block Trump's declaration, $3.6 billion of Trump's wall will come from 10 U.S. Code § 2808, or construction authority in the event of war or national emergency.

Congress has given the Department of Defense only two legal exceptions to the requirement that Congress maintain the power of the purse. First, it is permitted up to $50 million of pre-appropriated military construction funding for the purpose of national security. Second, it is given construction authority in the event of war or national emergency. For this second exception Trump is using, the law says, "the Secretary of Defense, without regard to any other provision of law, may undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces" if the president announces a national emergency.

The last two times a president has invoked Section 2808 were in 1990, responding to the Iraqi invasion of Kuwait, and 2001, responding to 9/11. In both of these cases, any military construction did support the emergencies that "require[d] the use of armed forces," a clear requirement for a president to invoke Section 2808. As John Yoo at National Review has noted, the courts could well allow Trump's declaration. Given that the border is under the jurisdiction of the Department of Homeland Security, not the DOD, it's clear that Trump's deployment of troops at the border provides him legal cover in invoking Section 2808.

But his national emergency declaration, while legal, clearly violates the intentions of the Founding Fathers in their delineation of powers of the purse and the president's explicit authority as commander-in-chief, not a unilateral legislator. This national emergency declaration, as the president has so much as already admitted, is a manipulation of already too-broad law to strongarm American taxpayers into funding something without congressional approval. The precedent this sets could theoretically allow the president to deploy troops in the name of climate change and allow the invocation of Section 2808 to construct physical portions of the Green New Deal. It's outlandish, but isn't using the same law President Bush used after a domestic terrorist attack to build a wall with no evidence that illegal border crossings have spiked catastrophically already beyond the pale?

[Read more: McConnell: Senate will vote to block Trump's emergency border wall]

Luckily a few brave, principled Republicans have already broken party line. Libertarian-leaning Sen. Rand Paul, R-Ky., who has come around to ardently championing key parts of the president's agenda, just announced he will join Sen. Susan Collins, R-Maine; Sen. Thom Tillis, R-N.C.; and Sen. Lisa Murkowski, R-Alaska, in voting with Democrats to overturn Trump's resolution. That pushes the total vote count to 51, meaning the bill will likely pass but not meet the two-thirds threshold required to make the vote veto-proof. Only about seven other Republicans have publicly come out as on the fence.

It's easy to condemn Trump's trashy tweets and sternly condemn the trade war that is only happening because Congress has abdicated its authority over trade powers for decades. But here is a moment for every Republican — especially those who the American people voted into office with the Tea Party promise that they would curb, not egregiously inflate, the powers of the president — to prove that principle and precedent matter more than a single, unconstitutional policy achievement.

This isn't just about Trump. It's about the office of the presidency as a whole, and a rare, vital opportunity for Congress to reassert its authority. The resolution is just one move to right decades of wrongs, and Congress must later go back and update the National Emergencies Act to clearly define and constrain the president's national emergency powers.

But this resolution is a start, and it's a stand against the catastrophe of executive overreach. Every Republican who identifies as a conservative should be supporting it.