"President Trump... pointed to nearly five dozen previous instances in which presidents of both parties have declared emergencies.... But there is no precedent for what he has just done."

Charlie Savage writes in the NYT.

None of the times emergency powers have been invoked since 1976, the year Congress enacted the National Emergencies Act, involved a president making an end run around lawmakers to spend money on a project they had decided against funding.

I don't think it's exactly true that Congress has decided against funding. They put some funding in the budget bill, there's been a lot of talk about deferring the issue of the wall, and there hasn't been any vote against the wall. So Savage is cheating right at the beginning of the argument. I think I can see that he wants to say that what Trump is doing is unlike all the other precedents, so he's characterizing it in a way that gets to that outcome.

Congress has... enacted a statute that gave presidents, in a declared emergency “that requires use of the armed forces,” the power to redirect military construction funds to build projects related to that use. It is that statute that Mr. Trump is relying upon, and his administration argues that this means he is exercising authority that lawmakers wanted the presidency to be able to wield.

But Elizabeth Goitein, who oversaw the Brennan Center study [of presidents' use of the emergency power, said]... “There is nothing approaching an ‘emergency’ in this situation, no matter how loose a definition you use.... And Congress has made it as clear as it can that it does not want the president to use funds for this purpose, so this is the president using emergency powers to thwart the will of Congress. That is very different from how emergency powers have been used in the past.”

She's relying to heavily on the notion that Congress has expressed its will and rejected the building of the wall. Where did that expression occur?

In a briefing with reporters on Friday, the White House identified only two previous instances in which presidents relied on emergency powers to spend funds on something different than what Congress had appropriated them for. Both involved military construction associated with wars: one under President George Bush’s Persian Gulf war emergency declaration, the other under President George W. Bush’s emergency declaration after the Sept. 11 attacks. Neither funded projects that Congress had previously weighed and rejected....

The budget bill contains $1.375 billion of funding for new border barriers, so where is the rejection? Is the simple failure to provide the full asked-for amount to count as a rejection of wall-building?

In the 1976 act, Congress turned off numerous old “emergencies” that had been lingering for many years and created a process presidents must follow when invoking such statutes. But the overhaul did not include defining limits on when a president could decide that a qualifying emergency existed, preserving White House flexibility.

Well, then, how do you stop Trump?

One check against abuse of that power eroded quickly: Congress had intended for lawmakers to have the power to overrule a president’s declaration by passing a resolution with a simple majority vote. After a 1983 Supreme Court ruling, however, presidents gained the power to veto such resolutions. That weakened Congress’s hand because it takes two-thirds of both chambers to override a veto....

They've had more than 30 years to repeal or rewrite the statute so that they're not giving so much away to the President. They tried to create a veto power for themselves, and that overstepped the Constitution. So the President has more power than Congress originally meant to give him, but he still has that power.

[S]everal legal experts said there was another possible long-term consequence that had received less discussion: by violating that norm of self-restraint, Mr. Trump may prompt Congress to eventually take back some of that power from the presidency — at least in a post-Trump era, when a succeeding president might be willing, or believe that it is politically necessary, to sign such a bill.

They're saying Presidents need to be careful using the power Congress gave them, or Congress might pass a new statute, taking back the power that they originally only meant to give while maintaining a veto power of their own. The Supreme Court told them they couldn't have the veto, and yet Congress hasn't revoked the never-intended presidential power. But it might, the legal experts said. It hasn't in 30+ years, but if Trump isn't careful, it might! So Trump should be careful. Okay, but he already chose to use the power, so it's too late for the wished-for self-restraint. And if you say that presidential self-restraint is the only limit, you concede that there isn't a legal ground to stop him.