But Elizabeth Goitein, who oversaw the Brennan Center study, pointed to the widespread dispute over whether, as a matter of empirical reality, there exists a true emergency on the border that would be resolved by a wall, as well as to the fact that Congress already made clear it did not intend to spend extra billions of dollars on Mr. Trump’s wall.

“There is nothing approaching an ‘emergency’ in this situation, no matter how loose a definition you use,” she said. “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.”

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 idea behind emergency powers laws is that Congress wanted the president to have standby authorities to activate in a crisis, when the government must respond quickly. The Brennan Center study identified 123 such statutes.

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.

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.