A week ago, Donald Trump for the first time publicly raised the prospect of building a wall through a “national emergency” declaration. The idea, which the president is apparently prepared to pursue, involves Trump granting himself emergency powers, borrowing the “power of the purse” the Constitution gives to Congress, and spending taxpayer money in defiance of lawmakers’ wishes.

I suggested at the time that Republicans may want to consider the implications of such a gambit. What happens, for example, when a Democratic president declares a national health care emergency and begins pursuing a Medicare-for-All system?

Some on the right have raised related questions. Erick Erickson, a prominent voice in conservative media, wrote on Twitter yesterday, “When the next Democratic president declares a national emergency over gun violence and takes executive actions to curtail gun purchases, you can thank the people urging Donald Trump to do the same with regards to the border.”

Sen. Marco Rubio (R-Fla.) raised the same concern on CNBC yesterday, though he pointed to a different issue.

The Florida Republican contended that Trump was elected on the promise of building a wall along the U.S.-Mexico border and the president has to “keep that promise.” But “we have to be careful about endorsing broad uses of executive power,” he added. “I’m not prepared to endorse that right now.” Such a declaration would set a precedent, Rubio said. “If today, the national emergency is border security … tomorrow the national emergency might be climate change.”

There are some statutory limits about emergency powers to consider, but broadly speaking, these conservatives are right to be concerned. Once the door is open, and presidents start pursuing their ambitions in defiance of Congress, it isn’t just Republicans who’ll walk through that opening.

In fact, Rubio’s example is of particular interest – since the climate crisis, unlike the need for a giant border wall, is real.

In fact, as NBC News’ Benjy Sarlin explained, if a Democratic president were to try such a move, “compared to the wall, there would be tons of government studies and testimony from [national security] officials backing up a crisis frame.”

Rep. Mo Brooks (R-Ala.) appeared on CNN this morning and was asked how he’d react if a Democratic president followed in Trump’s footsteps, tried to declare a “national emergency” on climate change, and began taking actions outside of congressional approval. The far-right Alabaman didn’t seem eager to talk about it.